If this has been covered previously please accept my apologies but I thought I'd share my experience of mounting filters on the LTM version of the Super Heliar 15mm f4.5. I recently purchased a mint condition copy for use on my M2 and CL.
I purchased a 48-58mm step ring and a 58-67 and for good measure a 58-72mm ring. I lined the innermost circumference of the 48-58mm with a length of 1.5mm width black medium density blackout foam ( originally purchased to fix a light leak in my Contax 139). This was then mounted over the fixed lens hood. It fits firmly and is a good balance with being easy enough to push on and being resistant to coming off. The 58-67 or 58-72 step up rings then screw on. 67mm and 72mm filters have been used without vignetting , nor with any apparent damage to filter surface.
The end result is non destructive, not a mark to be seen on the lens when the filter is removed, no light leak, no vignetting etc.
Now if only I could improve the suppplied viewfinder with a bit of black foam and two step up rings.
