That's a new-style switch, for those who haven't seen one. Actually, just the contact cartridge--it plugs into the greater switch assembly. It replaced the older simple, heavy one mainly for ease of automated assembly. Many more parts than the old one, but it seems to be faster to put together without lots of fiddly hand stuff. I'm guessing it's not used in the C-cells because they don't have the diamter to hold it, and the MC just needs a higher quality switch.