Some more investigation with a new automated test suite based on the 5101.1 names yielded two more duplicates, these also being present in MSN1:
Postfix and ISO C6/C5 (114x229mm)
PRC10 and ISO C3 (324x458mm)
I propose we drop Postfix and PRC10 from the standard self-describing names; they can still be aliases for the official names.
Michael Sweet, Senior Printing System Engineer, PWG Chair