This is the inner workings of your YashicaMat ."D" is the assembly that's wound on by the film crank .It drives "E" which turns the gear
directly above it , that is the gear connected to the end of your film roll via the "Pin" as you described it . To be able to wind on, "B" must
be in the position it now is (away from "D" , if it's closer to "D", then the end of "B" prevents the crank from turning by locking against the
tab you see on "D" which is just under the "B" tab.) It's easy to remove the plastic coatings and screws underneath , with the plate off this
is what you should see . Hope this helps a bit. I can supply more photos and possible explanation , others here I'm sure are well versed
on the workings of these cameras. Cheers, Peter