[Arran]

The new models were first introduced to Philco dealers in May, and to the public in June. So a May xx 1931 (or May xx '31) stamp is not out of the ordinary.

 Ron answered the question, and it makes perfect sense that they would do things that way so the local dealers could arrange displays and post ads in the local newspapers. There were some interesting discrepancies between cabinet production and chassis production as has been noted, like the early model 50 cabinet in another thread, which was a model 70/21 cabinet drilled to accept a model 50 chassis, as well as the assortment of chassis stuffed in grandfather clock cabinets, so clearly they were not always produced on the same timetable.
Regards
Arran