Upload & Sell: On
The ring looks far too well finished to be a one-off custom mod. I too would guess that's it's related to focussing, perhaps as part of some type of focal-pull device for a video rig, or maybe to interface with an underwater housing?
The index mark is also a curious feature. Does the ring rotate freely around the lens barrel, or is it fixed? If the latter, perhaps the usage scenario for some reason required the index on the lens body to be obscured. Are there any scuff or wear marks that might give a clue as to what was once connected to it?