[GreenKeys] Determining the age of a Model 15-KSR

[Jim Haynes]

I think holding and pulling magnet selectors were available at the same
time.

The single-magnet selector used in the 14s and 15s was supposedly
invented by Howard Krum when he was in New York and sketched the design
on a napkin at Coney Island.  This is patent 1,745,633 filed in 1924
and issued in 1930 and shows Sterling Morton as a co-inventor.

   Then the holding magnet selector was invented by Walt Zenner (patent 
1,937,376, filed for in 1932, issued in 1933).  The reason for it, as told 
to me by Walt himself, was to allow the voltage on the cord circuits of 
TWX manual switchboards to be reduced to 48V from 120V.  TWX operators 
were getting painful and dangerous shocks from coming in contact with the 
cord plugs.

So they were definitely still making pulling magnet selectors long after 
the holding magnet selector became available.

jhhaynes at earthlink dot net