When did zither builders switch from oblong tuning pins to the square tipped "zither pins" one finds today?

The zithers that I've seen from the latter part of the 1800s have these rectangular style tuning pins. It certainly causes a problem when the tuning key becomes disassociated from an instrument, as these are no longer made. Why squared pins gained favor, I do not know. It may have been a material and cost saving measure, but this is just a guess. Rather than a quick switch to square tipped pins, there was likely a gradual transition as a zither maker used up their stock and, if business was slow, this would have taken a number of years. I own a Schwarzer from 1912 that has rectangular pins. I suspect that a number of zithers made after this date do too.

I'm sure a better answer is out there, somewhere.