This rule I was completely unaware of until recently and then was delighted to
find such an heiress in the Kingmaker's pedigree. The rule is documented in
The Oxford Guide on pages 134 and 135.
Joan Brewer was a co-heir to William Brewer of Brewer and Torbay. She married as his first wife William Percy III of Topcliffe, Yorks. She and William had five children (of whom one is unknown) all girls and thus her heiresses. But they were not heiresses to William Percy as Joan died before him and William then had two more wives and five sons by his second wife.
One of the heiress-to-her-mother's daughters was Anastasia who married Ralph FitzRandolph. In order to recognise that she was heiress to Joan Brewer, herself an heiress, but not herself a Brewer, it is now the practice that such heiresses bear their mother's arms but with a canton of their father's arms which is then quartered by their descendants, thus: