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For the MOST part,, Yes,, all SUPER Single-Six's were convertable. The model designation is SC-4, SC-5, SC-6. The "S" stands for Super Single-Six and the "C" stands for Convertable.
However,, there were some (approximately 15 made,, but only one known) SC6M in 22 mag only. Very rare.
Most likely your gun was shipped as a convertable,, and the extra cylinder got misplaced away from it somewhere along the line. But,, the BEST way to figure out what you have would be to call Ruger Records,, and ask the nice ladies the model and shipping date. They use your serial number to look them up,, so have it handy.
I found the quoted post above from Contender, from March 2, 2011! So now I know what S (super single six) and C (convertible) are in my letter. What are the numbers designating? I got my Ruger letter yesterday, about my old Single Six. Thanks!