This is an interesting variation on the rotating lock for a hump filler. The external rotating lock has a partial spiral cutout which will push the hump away from the nib into a locked position when rotated. Please note the positive “stop” only when the hump is released. Rotating the collar into the locked position will get tight enough that it stops turning and hold the hump. No brand markings or patent dates. Missing the cap. No clues as to the maker.