Edit: question modified for future googlers after discovering it's an M not a W.
I recently bought one of the many varieties of tricked-up Helios you can find on eBay - fake anamorphic bokeh & purple flare.
It's also modded to focus to infinity on a Nikon F-mount.
It does pretty much what I expected it to do & I'm having fun making odd pics with it.
It runs fine in Manual mode on my D5500, aperture & focus rings doing as they should, plus 'guesswork' exposure.
One aspect has me puzzled, though.
On the underside there is a knurled knob with a red W [or M] on one smooth area. It only turns far enough to expose or hide that flat plane with the letter on it.
It doesn't appear to do anything, as far as I can tell.
Is it part of the original mount/clip/release mechanism, or does it serve some other purpose, even with the F-mount it now sports?