I have the Voigtlander Bessa III 667W (just arrived). Tried it with the Thumbs-Ups I have. The CSEP mod 1 felt tight on trying to fit it, so I didn't force it. The TU-CSEP-2 slipped in easily.
Because of the size of the camera, the thumb (well, my thumb anyway) falls most naturally against the rubberised grip on the camera back. The TU sits just a little higher than that, and if the thumb is placed into it, the index finger is forced to be slightly more crooked in order to press the shutter release. It's not uncomfortable, and I think it may be more stable for low-speed shots.
I hope that helps.
Voigtlander Bessa III 667 W
Leica III Black & Nickel