Some Herrett stocks have that partially exposed backstrap, and some Sanderson stocks as well -- but I don't know about those interior rounds. They look pretty custom to me, as VM observed.
The serial number we see would be found on a postwar N-frame, but the revolver itself looks like it might be a K-frame because of the lack of an ejector rod shroud. Can you identify the revolver for us, or at least specify the chambering? It's obviously a target model because of the grooved fore- and backstraps, as well as the adjustable rear sight. The hammer profile makes it a postwar transitional, but what appears to be a single-line address stamp says it was manufactured in early 1948 or before -- which doesn't completely square with the serial number.
EDITED TO ADD: Based on additional info below, I see that I read the stamped number on the stocks incorrectly. It makes perfect sense for that number to be on a late 1930s specimen of a K-22 Outdoorsman.