Hump fill with external cam lock

I saw this pen on ebay but I was outbid. No brand markings. Missing the cap.

There is a hump for filling. 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. I assume that rotating the collar into the locked position will get tight enough that it stops turning.