Yes, it is a IIIb. There is something coming out of the body to the left of the 9:00 position of hte lens mount. Looks like an allen-head screw. Could this be the flash synch Colyn wrote about? Pretty strange place for one--in the way of focusing the lens.
If it's a fake, it's a good one. The engraving looks right. The frame around the front finder window looks right--unlike a Russian fake. The body screws look right for a IIIb. And it has a round cam follower, not a Russian triangular one.
I agree, it's a real Leica with an aftermarket release button. But a IIIb, not a IIIa as the seller wrote. Can anyone make out the serial#? I can't.