There is no question that the gun is a "Mag Only" gun, but there are a few exceptions as is usual with Ruger. I offer the following info from Chad Hiddleson (Chet15) back in 2012 . . . emphasis added.
"Ruger’s Magnum Single-Six was introduced in July 1959 with .22 RF MAG rollmark on the left side of the cylinder frame and 6-1/2" barrel only.
"When it came out, quite a few customers began sending their WMR only Single-Sixes back to the factory to have the .22 LR cylinder fitted to them for a charge of $10 or $15.
"Likewise, several LR only Single-Six owners began sending their guns in to have the WMR cylinder fitted to those (but only to guns with sn's higher than 150000 since the bore diameter was increased by .002" with the introduction of the .22 WMR). These "converted" guns can be identified by the last three digits of the serial number stamped on the front of the "added" cylinder in large 3/32" digits.
"By March 1961, Ruger decided that a Single-Six with "convertible" cylinders would be popular, so they began offering it with all four barrel lengths, the first company to make a convertible model. But, Ruger left the same .22 RF MAG. CAL. rollmark on the 6-1/2" guns, considering the LR cylinder to be the "extra" cylinder. With the 4-5/8", 5-1/2" and 9-1/2" Single-Six Convertibles, the Mag. cylinder was considered the extra cylinder. In all four models, the "extra" cylinder was stamped with the last three digits of the serial number in small 1/16" digits.
"Month by month, a gradually higher percentage of Single-Sixes were requested by distributors to be convertibles, so in November 1962 by 340000 or so in the Magnum marked guns, Ruger decided to merge the 6-1/2" with the other Single-Six models and just make them all the same, eliminating the need for the Mag. rollmark on the 6-1/2" guns. When this occurred, only about 5% of Single-Sixes then being made were LR only, and Ruger decided at this time to only offer an LR only model in the 5-1/2" which continued to be cataloged until 1972.
"So, even after the change in rollmarking for the 6-1/2", the LR cylinder was still considered the extra cylinder (as a employee workbench procedure) and the Mag. cylinder was considered the "extra" for the other three barrel lengths, so only this extra cylinder was still stamped with the last three digits of the serial number.
"This all changed about April 1963 when Ruger got ahold of an electric pencil, which probably let an employee scratch the front face of both cylinders (with the last three digits of the serial number) in half the time it took to stamp one cylinder with three separate number stamps. This is also when Ruger began printing in their Instruction manuals that both cylinders must match the serial number of the gun.
"Even though the Magnum Single-Six (.22 Mag. only) was dropped from Ruger's catalog and price lists in 1963, there were a very few Mag. only Single-Sixes shipped from the factory without the .22 RF Mag. rollmarking. These are known from 342128 through 398349 in four different variations, and all are very rare!"
