richray,
Yes.... there were indeed at least 15 of the model SC6 Super Single-Six with 6-1/2" barrel shipped from the factory as .22 Mag. only... these were actually catalog number SC6M. The only one that had been known was 555430, however we have no idea of the whereabouts of that revolver today.
That being said, the maker of Ruger's two piece boxes also made a printing error, and it just happened to occur at about the same time the SC6M were produced. In the 560xxx to 566xxx range the SC6 box started appearing with ".22 W.M.R. CALIBER REVOLVER" on the end instead of ".22 CALIBER REVOLVER". A few earlier than that serial number range are also known to have been shipped in that box as are a very few with a 60- prefix.
At the very same time, Ruger's RSSM printed 2-piece box for the fixed sight model began appearing with ".22 CALIBER REVOLVER" printed on the end instead of ".22 W.M.R. CALIBER REVOLVER" which had been printed with that since the mag. only guns came out in 1959. There are also a very few of those known in the early 20- prefix.
So I really think Ruger's box printer got their line-o-type mixed up between box printings.
So next, does your gun have only a mag. cylinder with it or is it a convertible? If it was shipped as Mag. only, the last three digits of the serial number should not be scratched on the front face of your cylinder. If it was shipped as a convertible, then Ruger scratched the front of both cylinders with the last three digits.
But if it only has a Mag. with it, I would still contact the factory to see if it was shipped as SC6 or SC6M.
At least 15 SC6M were produced for a distributor. That invoice stated it was part of a larger order of say 30 or 35 guns (I don't remember now).
Let us know how it turns out!! If it comes back as SC6M you have a unicorn!!!
Chet15