Hey, i have an Edwards 1521, but it has a weird switch on it. This was used (in the 70s) with edwards 360-A flush mount horns. Those were left abandoned in the wall and upgraded to spectrealert classic devices. Anyways, this 1251-0A does not have a glass rod sensor. It has a button switch that’s open when the door is not pulled but closed when it’s pulled. I have no idea what this is for since if you pull it in the first place, it will spin the code wheel and set off the system.
That appears to be an aftermarket button, which might have been put there to convert the station from coded to non-coded: an actual 1251 tamper switch is right where the hole in the front of the handle is (next to the protrusion that breaks the glass rod), which likely throws a trouble at the panel if the glass rod goes missing from its holder for any reason.