I got this 2903 a long time ago, and me and a fellow enthusiast were chatting about it (he owns many vintage simplex alarms) He brought up that mine might be a rare variant/early or late version, since mine has a rounded square hole rather than the usual octagon hole. If anyone knows anything about this please tell me, thanks!
I know someone once told me what generations have what shaped-hole, but I forget. Such versions probably are rare though I’d imagine.