I also have a rebranded EST System Sensor http://jjinc24.info/collection/est/2551f smoke detector. It appears as though EST and System Sensor did have some sort of rebranding agreement at one time…
Also, while I was resetting the BG-10, I accidentally stopped applying pressure to it while screwing back in the hex screw, and I had it screwed in just enough so that the pull station wouldn’t fly open, but not enough to prevent the switch from reactivating.