Sorry, I didnt know how to size it down for the site. It sounds like there is not much collectability for an avaerage condition gun.Some of the stuff on the screws may be greaseas its been in storage for awhile. So the cylinder is somethingsomeone has addedor had added at the factory.
The Magnum Cylinder (value $25-$50) should not be used in that gun, Barrel is sorta small, yea , yea I know it will work, I highly doubt it is accurate.
Collector; probably not, Shooter, YES and a NEAT old 3 screw original gun. My opinion, I would rather have the OM than a NM.
Value end user might go $400 I see it more around $300 in my neck of the woods.
Here is it's near cousin aquired this July this year, with tax at Retail.
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.