Afraid you are on your own regarding the rear sight's graduations, as they are a trial and error sort of thing.
Would suggest shooting it at its lowest setting at a moderate distance and go from there. These guns were not serial numbered
(S/N's were not required until pasaage of GCA 68), but there may be a barrel code in an oval. One of the knowledgeable folks here should be able to interpret it for you. I had a 325 a long time ago, but never bothered to pay much attention to it. Never figured them to be a possible sleeper...
