Yes, Ruger wouldn't fit a Mag. cylinder to a Single-Six serial numbered under 150000. Through the years they may have forgotten this rule and fitted a few, but the bores are too small for those earlier guns.
You can check to see if it has been back to the factory by looking at the end of that mag. cylinder to see if the last three digits are stamped on the front (should be large 1/8" numbers). If not, the cylinder has probably been added by somebody outside the factory.
Chet15