1959 Tenite and ABS switch over sets

[unbeldi]

My first inclination about the mechanism of applying the "molded" date stamps inside the caps is that they were not part of the injection mold until the late 1960s, but I don't have the volume of data to say which year.
I have some black caps made in 1969 that have a date "clock", with tick marks, similar to the housings.
Early in the decade and for those still in the 50s, I think they used a stamping tool that was heated and impressed the date into the plastic after molding. This requires very little heat and can look very precise, but most date actually are not so sharply defined.

These dates are impressed into the plastic, not raised above the surface. To contrast, there is another larger number molded into many of them, perhaps a mold number, that is raised, not impressed.

There appears very little "standardization" of these date stamps in the caps. I am going to show some more, among them a turquoise cap with the same date stamped twice, perhaps indicating that this was still a manual process.

Certainly all cap molds appear to have been replaced at the end of 1959, because of the center hole that appeared right with the start of 1960.  Has anyone seen a 1959-dated cap with center hole?  I don't think I have.

I think all the 500-series consumer telephone sets were made in Indianapolis starting in 1950, and Princess set in 1959. The Princesses were much later also made in Shreveport.


PS: here are the caps:

Both turquoise caps have a raised "C" molded into the inside surface, hard to see, while the dates are clearly hot-stamped.

The black caps have a completely new dating format, which appears entirely mechanized, and they are all raised.