- Revisiting the Cedar Knolls Telephome Museum I found a photo of a 111A2 telegraph relay test set that I worked on in the 1950's:
At that time a big city CO had a one or two thousand 209,215 & 255 (plug in) telegraph polar relays(dating from the late 1920's) in service;these required a lot of maintenance for dirty/pitted contacts, pole alignment & armature replacement.
Virtually every TTY circuit up to the early 1960's went through one or more of these relays.
A 255 polar relay is shown on this page:
The Signal Corps had a portable version known as a 193-A test set shown here: