Amseco is the original manufacturer of these bells (there’s an Amseco logo at the back of the base); they were also rebranded by Simplex. I think the main advantage of these motor bells is that they have a much lower current draw than traditional vibrating bells like the 439D. 439Ds are gluttonous, consuming a whopping 0.085A of current; this MB6-24, on the other hand, only draws 0.025A. The mechanism in these bells features a rather unique design, as shown in this video.
Fortunately, I already have some homemade keys that work well. I still wish I could find an original key, but I know they’re hard to find (and I’m certainly not the only collector who’s looking for one!).
I had always wondered who made those bells, and I never found an answer until I received this one. It seems that System Sensor is indeed the manufacturer: this bell features a quality control sticker with System Sensor’s logo at the back of the base. I see the Edwards- and Notifier-branded versions of these bells far more often than the Siemens-branded models.
Interestingly, this bell is a nearly perfect clone of an older Amseco design that was common in the '90s. The photo below shows the Siemens bell (at the left) next to an older Mircom-branded Amseco bell (the Amseco logo is visible in the upper right corner of the base). Despite the similarities, the gongs are not interchangeable due to a slightly different alignment pattern, and the motors/mechanisms appear to be different as the Siemens/System Sensor bell has a higher current draw and a longer striker.
The hole to insert a screwdriver is actually on the left side of the pull station. The mechanism in these pull stations is completely different from that of the current MS-401-style pull stations: pulling the front cover releases a spring which, in turn, presses down on a switch as it swings downward. This video demonstrates the operation of these pull stations.