The .45 was a clone of the OS .45 FSR. I think most parts interchange between the two. The Magnum .46 Piston/liner set will fit in the present OS .46 FX also. Probably one reason Magnums are cheaper, less money spent on R+D. They are slightly heavier than the new OS/Magnums, but many people think the FSR's were the best OS engines ever as they had no liner peeling issues. I've had a couple Magnums and they perfect runners. If it's in good shape- run it! Ross