A generalization dug out of my data base FWIW (subject to speculation)...
Ruger Single-Six 22LR Only: The very first standard production SS's (starting in 1953) were all 22LR only,
Cat. No RSS5, and were a steel frame/alloy XR-3 grip frame with a flat loading gate, checkered hard rubber grips, a drift fixed rear sight, with 5 1/2" barrels,
and with "Ruger 22LR Single-Six" marked on the LS of the frame. Ruger did also offer stag and ivory grips from the start, and varnished walnut from 1955, all with black eagle logos as accessory items for them until about 1962 (stag), 1958 (ivory) and in 1960 walnut became the standard with the hard rubber grips changed to an accessory item. In about 1957 the loading gates changed to a roundgate. Alloy frame 22LR only versions (called Lightweights) with alloy or steel cylinders, 4 5/8 barrels in tricolor (flatgates) and blue (roundgates) were also offered from 1956-1958, each version in its own serial range. Very late in their production 4 5/8 and 9 1/2 barrels came along. too much to list, but I have some serial number ranges...
The 22MAG Only SS's came out right after Win introduced the 22MAG, and were
Cat. No. RSSM. So they were made from 1959-1962, had a serial range of 300000 to 340XXX, are
marked "Ruger Single-Six, Win 22 RF MAG Cal" with 2 lines on the frame (left side), had hard rubber grips only as standard the first year but they added walnut the second, they have round loading gates and all of them have 6 1/2" barrels. The last year or so they also made and shipped some of them with an extra 22LR cylinder. Even so a later old model SS convertable 6 1/2" with only a 22MAG cylinder wouldn't be confused with the earlier 22MAG Only revolvers due to the frame stamp and the serial number. Kind of strange... these newer RSS6 convertables where shipped in boxes still marked RSSM to the end of their run in 1972 (Ruger believes in waste not, want not I guess).
When the old model SS's actually first started being shipped with dual cylinders is laced with a lot of speculation. There are lots of different sources of that information published by so called experts, but they don't agree with each other and the spread between the start dates can be counted in years, not just months. Typical of why the data base I assembled can only be a generalization. Anyway, a call to Ruger and someone in customer service that knows for sure or will check records is the definitive answer to anything about the old model Rugers.
HTH
L.