I have three C3's , a Matchmatic and two older black ones.
The lens glass appears to be the same diameter, but the barrels are different.
The Matchmatic seems to use Series V filters and the barrel is threaded with the Series V retaining ring.
The older ones appear to use a Series IV screw-in filter holder, or a filter mentioned above.
Can anyone identify the thread of the smaller variety of lens barrel? I have a box of Series IV and V adapters somewhere and if I knew what I was looking for it would be easier...
The two older ones came with two different manuals with early-mid 50's copyrights a couple years apart. The manuals are very different and even have a different model holding the camera.