This is a M8x.75 (female thread) to 1/2x28 (male thread) thread adapter with female 1/2x28 thread protector.
This is great for older Walter and S&W .22lr pistols that have threaded barrels that do not protrude in front of the slide.
However, this will work with .22lr barrels threaded with male M8x.75 threads as a thread adapter to 1/2x28 male threads. 1/2x28 is the most common thread pitch for .22lr suppressors and muzzle devices.