I found an unissued .45 Colt cylinder for my M25-2 on this board many years ago. I presume it was originally produced by S&W for the -3 guns, and this was some sort of production over run, much like the .22LR cylinders for the M53 that were available once but eventually disappeared along with the mastodons and passenger pidgeons.
Magically, the 45 Colt cylinders were made to fit the gun, so no alteration of the gun itself is really necessary, and I was able to send it back to the mother ship for proper installation. The frame lug stays the same for both cylinders, which are the same length.
The .45 ACP cylinders for the M25-2 pop up on ebay fairly often, and don't cost anything like what the -3 cyliiders would cost even if one could be found. Mine is the only one I've ever seen for sale, and I haven't seen another since.
The convertable M25-2 shown below with my convertable M48.