I put my C lens in the module of my first Mobius (A lens). The images look great and even with outside temperatures of 29 Celsius degrees there was no focus shifting (in 2 hours recording) as with the original module (first iteration of FRP modules).
Screwing the C lens was very easy all the way, but I felt there is a little movement when I tighten the screw to keep it in place.
Screwing the A lens in the FRP module was pretty difficult from the begining and I didn't manage to turn it enough by hand to get a decent focus, so I gave up for the moment.
Also I noticed that the A lens had some sticky grease at the end of the barel, while the C lens didn't. Looks like it's for higher friction, in order not to turn too freely when you adjust the focus.