Welcome to the Forum, I agree with 'street' and as far as we know the OM "Magnum" (only) guns supposedly stopped as such in 1964, as RENE had noted ,none made after that....as far as yours and it being a 'prefix' gun came after the 1968 GCA and Rugers started to use a prefix on all of their models......"single" cylinder guns usually had a marking onder the cylinder frame ,up front) with an 'asterisk' or "star" or an "arrow" (some say it looks more like a house, I do...) and these single cylinder guns are "long rifle" cylinders...bottom line it would take a "letter" to find out just what it may have shipped as , and its "catalog' number they assigned to it..........
both sound like nice guns, and the alphabet Bearcats are pretty good "finds"...good luck