I found a home for the 500DM rotary desk set. Thanks to all who pm'd me.

Also, for what it is worth; When we installed an old pre polarity guard TT phone we found that when the T and R were reversed and you held the hand set receiver to your ear you would not hear any TT's as you pushed the buttons and of course you couldn't break dial tone. If the line polarity was correct and you’re heard the TT's but couldn't break dial tone then our friends at Telco hadn't translated the line for TT.

Forty six years and still fascinated with Telecommunications!