The .22 caliber adaptors were built and marketed by Parker-Hale, patented 17 January 1923, "supplied in quantities to the Canadian Defence Department, the Small Arms School Pachmarchi, India and the Government of Northern Ireland. By 1940 is was also available for use with the caliber .38 Enfield Service Revolver and the .38 Webley Mark IV. (pg. 232, Webley Revolvers, Bruce & Reinhart, 1988.)
Pretty scarce, actually. The kit was intended to temporarily replace the regular cylinder for economical target practice. So the revolver up on auction is "missing" the original cylinder.
I did a fast skim of all of my Webley books. If looks like P-H did conversion kits for both 4" and 6" barreled revolvers. Interestingly, I have one that will fit either - the tube extension simply threads into the shorter tube. Webley did do a single-shot mono-block conversion kit, which I presume was for Webley pistols. The 1945 Parker-Hale catalog has an array of .22 caliber training devices, including one for the 2-PDR artillery, the Boyd anti-tank rifle, the Vickers Machine Gun and the Lewis Machine Gun.
(Note: pop over to Cornell Publications They have reprints of many old gun catalogs. There is an amazing amount of information to be found for very few $$.)
I've seen the Parker-Hale Webley conversion kits go for $400 - $1200. Revolver not included.
Yeah it's a webley with a 22 conversion kit installed.
The barrel has threading on the end because it is a sleeve that is inserted into the existing barrel, then threaded to be tightened. If you have the original cylinder, you can swap between 22 rimfire and 455 webley.
These are extremely desireable - especially if you have the original cylinder - because you retain the ability to shoot both original 455 and cheap 22 lr.
Parker Hale would do the fitting if you sent your revolver to them. A Parker Hale fitted unit will have the last three digits of the revolvers serial number stamped on the outside the the .22 cylinder. If you get a .22 PH unit without the revolver, the unit might not fit your revolver without doing some work. The real test of a revolver smith is making the PH unit fit and function while making sure that the original .455 or .380 cylinder still functions correctly.