The curious things about these bookplates (they are identical, both off the same press) are:

1. The surname is spelt wrongly, with an 'i'
2. The helmet is open and forward facing, that of a knight's
3. The name is just "Philip Powys", no second forename of "Lybbe"
4. There is no motto.

First it is likely that this spelling error is by the engravers, though this spelling was used by others in the 17th century.

Second I suspect that this bookplate was copied from one for, or from the arms of, Sir Thomas Powys, father of the first Philip Powys (I wonder if he was not named after his maternal grandfather, Sir Philip Meadows?). Sir Thomas was a knight of course so this open, facing helm would have been correct.

Third, in all formal documents, the Powyses with a Lybbe forename always used it. There were no Philip Powyses without Lybbe as a forename except, of course, the one who married Isabella Lybbe.

Fourth mottoes were a late feature of heraldry and may well not have been used by early Powyses.

Further I surmise that this pair was a printer's proof and was probably rejected because of the two errors, the knight's helm and the misspelt surname. But they have survived for three centuries!