My understanding is:

before early 1970s, FAA hadn't standardized the manuals into a POH format
yet, so they were just Owner's manuals at the manufacturer's discretion.

Did the 1st page after the cover still have to have the serial # on it?

I fly an old 150F model here lately and it's Owner's manual is hardly up to
standard that I'm accustomed to. Harmful, I don't think so, but I'm after
legalities here for knowledge sake.

thanks,
ck