The plastic cover over the switch hook contacts has been present since the early 500 sets on all brands. Most items that were changed to plastic were the frame of the ringer assembly and the network. Instead of a network enclosed in a potted metal can, ITT, S/C and Northern Telecom switched to an open PC board with a plastic base to secure it.

These made the phones much less resistant to damage when they were dropped. WE kept their network in the metal frame/enclosure forever until very near the end of the manufacture of these sets.

This all had to do with saving money PLUS the fact that telcos didn't own the sets anymore, so they didn't care if they broke. When they were being rented, that was another story as Skip mentioned.

Ed Vaughn, MBSWWYPBX