Been a while since I reviewed this thread, but was contacted by a member who suggested that maybe the changeover from stamped number to foil sticker is connected with the manufacturing plant that the wiper motor assemblies came from.
I hadn't spotted this before, but the motor casings appear to have 2 different stampings after the 'DELCO PRODUCTS':
Either 'DAYTON OHIO, USA' or 'ROCHESTER, N.Y. USA'
From the limited data points in the last updated table, it's impossible to identify when the stickers were actually introduced, but it would appear that there was a transition during the model year, possibly something like this:
Stamped P/N only (early)---------Stamped P/N and Sticker (mid)------------Sticker only (late)
Wonder if wiper motor production was switched from Dayton to Rochester during production, or if both manufactured motor assemblies throughout?