From the standpoint of felt recoil, you've got to love the extra heft of a steel frame, whether it be a 39, 539, or 639. To add even a bit more weight, you need one of these - a steel backstrap. These were made for the Model 52, albeit with a silly tab sticking out at the base that on a 39 series gun would be the lanyard loop. In this case, years ago I went to a local machine shop and had the center of the tab milled out. I also used one of these steel backstraps in putting together my
Frankennine, although for that gun I removed the tab altogether.