Here’s some content: Today I got 7 pull stations from a person in a undisclosed location. I got 5 Simplex 4251-113s, 1 Simplex 4251-13, and 1 4251-21 (it has a metal button). I wonder what’s the difference between a Simplex 4251-13 and a 4251-113.
That is a question I am qualified to answer via an eBay seller that didn’t know the difference, either!
The 4251-113 has a D.P.S.T. Mercury-bulb switch, the back having a black cover to protect it, and was used primarily to connect to an annunciator and the main F.A.C.P.
The 4251-13 has an S.P.S.T. switch, so my theory is that the number of 1s dictates S.P.S.T. or D.P.S.T.
Glad to help,