Please report back what you found. I too have this lens and have used it on my S2 ( long since sold ). I just picked up a Contax to LTM adapter and found the "ring with ears" does in fact rub around in the inside of my IIIc body. I removed the ring, but just realized it may be there to protect the rear element, but why is it shaped the way it is?
I can use the 21 on my new to me S3 2000, but I like the compactness of the IIIc and found this adapter, so I'm out today shooting a roll to see how it works.
Anyway, I'd be curious too to know why it is hitting the inside of your camera.