The following was provided to me by Irene Szymanski who analyses finds shown to the Portable Antiquities Scheme of the British Museum.  (These words are needed to disabuse any readers from the thought that I have done such eminent scholarship).

Guillaume de Grandison

Enamelled copper-alloy horse-harness pendant; traces of blue and red enamel remaining.Found near Ferriby (Hull). Less than 1" maximum dimension. Although it is impossible to be certain of the original charges on the bend (diagonal band) on this pendant, the apparent head and slight frilling of the edge suggest that they may be birds. If this is the case, then the pendant displays the arms of the Savoyard knight Guillaume de Grandison of Ashperton, Herefordshire: paly argent and azure, on a bend gules three eagles or.

Although less well-known than his older brother, Otes (Otto) de Grandison, a valued friend and companion of Edward I, who featured prominently not only in the conquest and subsequent rule of Wales at the end of the 13th century, but as a diplomat in Edward's service, Guillaume was a man of considerable standing in his own right. The son of Pierre, sire de Grandison, he married Sybilla de Tregoz, co-heiress of her father's fortune and so acquired considerable wealth. He served as his brother's deputy in Wales and the Channel Islands in the period 1288-1295 (Otes being lord of the Channel Isles for life and Justiciar of North Wales), and from 1288 he supervised the construction of Caernarfon Castle. Perhaps surprisingly, the family made no attempt to establish themselves as a new aristocracy in Wales, but were content with high office and went on to serve in Edward's Scottish campaigns. Guillaume died in 1335, leaving as his heir his son Piers; the latter married Blanche de Mortimer, Roger de Mortimer's daughter. Piers is recorded as using his father's arms when he fought against Edward II at Boroughbridge; he was captured and fined, as well as being required to give sureties of good behaviour. He also sealed with these arms (untinctured, naturally) in 1353. Piers himself died without issue in 1358, and his tomb is at Much Marcle in Herefordshire; his father's effigy is in Hereford Cathedral. IHS

10/08/00GRANDISON, William de F 358, LM 310: [No tincture], on a bend gules three eagles displayed or. H 90, K 34, GA 92, SP 111: Paly argent and azure, on a bend gules three eagles displayed or (also N 97). William de Grandison of Ashperton, Here., a younger s. of Pierre, sire de Grandson (d. 1258), and a younger bro. of Otto de Grandison (d. 1328; see above), m. Sibyl (dau. and coh. of John de Tregoz [d. 1300]), served as Otto‘s deputy in Wales and in the Channel Islands, 1288-95, supervised the construction of Caernarfon Castle beginning in 1288, summoned to serve against the Scots, 1299, 1300, and frequently thereafter, d. 1335, leaving a s. and h. Piers (d.s.p. 1358). (Parl. Writs 318, no. 2; 337, no. 20; 348, no. 3; 366, no. 1; 377, no. 22; Cal. Inq. Edw. III 7: 460-61, no. 676; CP 6: 60-62; Galbreath 1927, pp. 58-60; Moor 2: 138-39; Sanders, p. 43; Clifford, A Knight of Great Renown, pp. 112, 264.) F and LM are defective as the arms are plainly those of Grandison differenced. The surname in F has been emended. Cf. the addition in Qe and Qf (ed. Mitchell, QII 489). William‘s effigy is in Hereford Cathedral (Mayo N, p. 15). For Caernarfon Castle, see Prestwich, Edward I, p. 214 (photograph between pp. 304-05, fig. 11; ground plan, p. 213); see also Age of Chivalry, p. 63 and fig. 33. Piers fought against the king at Boroughbridge, 1322 (CP 6: 62-63; Moor 2: 137), where he bore his f.‘s arms (O 208: Paly argent and azure, on a bend gules three eagles displayed or). Piers‘ arms and those of his w. Blanche (dau. of Roger de Mortimer, earl of March [d. 1330]) are on their tomb in the church of Much Marcle, Here. (Wilmott 1988, p. 113). William sealed with a paly and three eagles charging a bend, 1283 (Birch 10210; ISV, p. 75, no. 2), as did Piers, 1353 (Birch 10201; ISV, p. 75, no. 3); Sibyl‘s seal, c. 1286 and 1339, displays the arms of her husband (Grandison), her father (Tregoz), and her mother (FitzWarin) in a trefoil (Birch 10203; Galbreath 1927, p. 60, fig. 62; Galbreath, Manuel, p. 230, fig. 654; Ellis P1463). Galbreath 1927, p. 61, also ascribes a fourteenth-century armorial window at Exeter Cathedral (fig. 64) to Piers. For another s., John de Grandison, bishop of Exeter 1327-1369, a great patron of the arts who bore Paly argent and azure, on a bend gules a mitre argent between two eagles displayed or, see Galbreath 1927, pp. 61-63; Gayre, Heraldic Cadency, pp. 38 and 39, fig. 58; and Age of Chivalry, pp. 230, no. 95; 463-67, nos. 589-98 [for the ivory triptych, dated c. 1330-40 (= no. 593), see also Marks and Payne 18]; 482, no. 638; (all references for John, except Marks and Payne, are illustrated).

Abbreviations:
F: Charles' Roll
GA: Galloway Roll
H: Falkirk Roll
K: Caerlaverock Roll
LM: Lord Marshall's Roll
N:Parliamentary Roll
O: Boroughbridge Roll
Qe, Qf: Collins' Roll
SP: Smallpece's Roll
(All in Rolls of Arms of Edward I (1272-1307) ed. Gerard J. Brault, Boydell Press for the Society of Antiquaries of London, 1997)

d.s.p.: decessit sine prole (died without issue)

Cal. Inq. Edw. III : Calendar of the Inquisitions Post Mortem etc., London HMSO 1904-13.
CP : Cockayne, Complete PeerageParl.
Writs: F Palgrave, ed., Great Britain. Parliamentary Writs and Writs of Military Summons, 2 vols, London, 1827-34
Age of Chivalry: Alexander & Binski (eds) Age of Chivalry: Art in Plantagenet England, 1200-1400, London: Royal Academy of Arts, 1987 (Catalogue of the exhibition)
Birch, Walter de Gray, Catalogue of Seals in the Department of Manuscripts in the British Museum, 6 vol, London, 1887-1900
Clifford, Esther Rowland, A Knight of Great Renown: the Life and Times of Othon de Grandson, Chicago: University of Chicago Press, 1961
Galbreath AV: Galbreath, Donald Lindsay, Armorial Vaudois, 2 vols, Baugy, 1934-36
Galbreath, Manuel: Galbreath, Donald Lindsay, Manuel du blazon, rev. ed. Léon Jéquier, Lausanne: Spes. 1977
Gayre, Robert,  Heraldic Cadency etc.., London: Faber & Faber, 196
1Marks, Richard and Payne, Ann (eds) British Heraldry from its Origins to c. 1800, London: British Museum Publications, 1978
Mitchell, Robert W., (ed.) Mimeographed and lists of Medieval English Rolls of Arms.....,
Peebles: Heraldry Society of Scotland, 1982
Moor, Charles, Knights of Edward I, 5 vols, Publications of the Harleian Society, 80-84, London: 1929-32
Sanders, Ivor J., English Baronies: A Study of Their Origin and Descent 1086-1327, Oxford: Clarendon Press, 1960
Prestwich, Michael,  Edward I, English Monarchs, 1988; paperback ed. London: Methuen, 1990
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In Moor’s ‘Kts of Ed I’, Vol II, pp. 138-9, it gives him a wife Isabella c. 1288 and in 1328 a wife Joan.  Who is this Joan?
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Paly of six argent and gules, on a bend of the last, three eagles displayed or. (Fox-Davies in Armorial Families, 7th Edition.)

This is undoubtedly wrong.  All the early records, including the Dictionary of British Arms, give a paly of argent and azure.  See Notes, as does the C of A Achievement for RCLPL.

Paxton book of arms:
Paly of eight Azure and Argent a bend Gules charged with three eagles displayed Or.

Paly of eight Azure and Argent a bend Gules charged with three eagles displayed Or.

1. RCLPL’s Achievement of 1927 by C of A,
2. Dictionary of British Arms, Medieval ordinary, vol 2, p. 11,
3. CP VI, p. 61, note (b),
4. Paston book of arms, M St thesis by Sebastian Nelson, p, 13.