My eyes aren't what they used to be, but if I am reading the headstamp correctly, it is a Peters H.V. line of shells (with Made in USA on it). If I am right on what I am reading, it will date from 1927/32-1960's. I have yet to be able to nail down the exact beginning year as different sources say different things. It seems that the H.V. line made it to the change from paper hulls to plastic hulls. It is a fairly common headstamp to find and was a very common hunting/sporting round. I would think to be able to tie it to being a prison guard weapon, it would have to have been found on a prison grounds that was in operation during those years, and even then that would still be an assumption.
Nice find though.
Doug