Ok,, I'll jump in here with a little info to try & clear up any confusion.
First off,,, while hittman has posted the info on the Ruger factory website,, it can often be incorrect, or a bit mis-leading. Yes,, the factory stuff has been known to be wrong.
However,, using the "Red Eagle News Exchange Reference Guide" which was published by Chad Hiddleson,, and what we collectors contribute to & use as a bible of info, here's what it says.
Ruger OM Magnum marked Single-Sixes which were marked on the frame with "Ruger Single-Six Win .22 RF Mag Cal." ,, and were made from 1959 until 1964. The serial number range was from 300000 to 398349.
Model info; RSSM
However,, during the 6 years of production, Ruger started offering them as convertibles.
That model had an extra .22 LR cylinder, and is model RSSMX. The X being for "convertible."
In 1961,, Ruger also started offering the OM Single-Six Convertibles. The were marked on the frame "Ruger .22 cal. Single-Six." Their serial number started in 1961 with 162xxx and went to 1973 ending with serial number 21-56105.
In that era,, there was also a model known as "RSSMX."
Confused yet?
Due to the way the .22 mag caliber was introduced in 1958,, and Ruger wanting to get into that market,, they made magnum only guns. But as time went on,, they eventually dropped the marking of the frame as a magnum,, since so many requests were for a convertible.
So,, the gun in question here is not a "Magnum" marked,, gun,, but it was still a convertible one. Built AFTER Ruger stopped making the magnum marked guns.