OK- the chamber cast is in the original thread. It has a 12.6MM base diameter and a 12.16mm shoulder. That's .496" base and .479" shoulder.
Those measurements are WAY larger than a .22 HP.
Back at the turn of the century, .22 cal barrels were .223" at MOST.
The .22 Savage was an anomaly with it's .228" bullet. Seems to me, the .22 WCF was also .228" but when Townsend Whelen and ?? brought it out as a smokeless powder ctg. it became the .22 Hornet with a .223" bullet, same as the later, original .220 Swift.
I just measured 4, .45/70 cases(for my lever actioned elephant rifle) on my loading bench and every one was .497" just ahead of the rim.
This should be the image link.Link doesn't work, but - the chamber case is in the article. by the looks of it(the case mouth and neck length), it is a holder for a .22 RF ctg.

https://www.gunboards.com/cdn-cgi/image/format=auto,onerror=redirect,width=1920,height=1920,fit=scale-down/https://www.gunboards.com/attachments/steyr-15-jpg.4279148/

Meant for something like this?
https://www.gunboards.com/attachments/img_3433-gif.4279880/