Welcome to the world of M2. The little flat screw or cap is there to hold the top-plate in place.
As for the Rapidwinder M2, the original series of 300 is sold out, but by popular demand (and to replace the ones i took off my own cameras out of the goodness of my heart) I have another series of 100 going. I added the feature of this being able to open/close the leica reloadable cassettes. It works fine with regular film cassettes of course, just like a regualr baseplate. I tend to use "long reel" film, either 100ft or 400ft rolls and the metal IXMOO's are good for loading as they dont scratch the film and until you have figured out how to, you will also spend quite a while in total darkness and not being able to spend money.
I still have some M2 Rapidwinders in stock (two at the moment) and enough stock to make some more. The original leicavit MP is rare and expensive, and parts are unavailable.
The Rapidwinder is designed primarily for those of us who focus with our left eye. It allows you to shoot several frames without dropping the camera from you face and it also avoids having the glasses skewed on your face after having them banged into the film advance arm!
Beware though, M2's are addictive and might lead to things like "Oh, i need one more for the 35" or "if I get a 21 it would be nice to have a dedicated body for it"
or the more common one "damn, I just want more of them"!