Thanks for the image. Yep, that's a flange.

If you use any of my KB stories, fine you're welcome to, but he was a fine economist, not a lawyer.

Take the flange off, turn it over so that the wide flat end oges onto the shutter first then screw it down tight agains the lens board. Now test the rear lens element and see if it will attach properly.

Thought about doing what 45PSS suggests, as the threades might catch in that direction, but I'm afraid that tightening the rear cell of the lens against the threaded side of the flange will damage the housing on the rear cell of the lens as the threaded side of the flange is not smooth. Is this not a concern?

Seems to me that its too long. If it is too long facing north, it will also be too long facing south. Get a retaining ring and put yourself out of your misery.

FWIW that looks to me like an adapter to put a screw mount lens for a 35mm camera on a 35mm camera body with a larger mount flange thread, such as to mount a Pentax M42 mount lens on a camera with an M45 mounting flange.
Twenty or thirty years ago various adapters of this type were quite common. A few months ago I sold on eBay an adapter to mount M42 lenses on a Minolta manual focus bayonet mount body. It looked quite similar except it had the bayonet mount on the outside instead of screw threads.
I concur with Dan's recommendation that you purchase a flat retainer ring.

C.Henry