Mr. Bieser wrote:Some models seem to have many distinct variations...
Or later update(s). I know of a wholesale update to one line circa 1926/27.
And there is a curious image in the introduction of MB's catalogue--it is an oblique image of an actual page in the original Master Book. You can see on that model what I think are 2-3 visible layers of electros (watch images) stacked on each other. So if that MB was ever forensically analyzed, we might have more info on transitions