As a rule, most single cylinder guns don't have the last 3 digits of their serial number stamped or scratched on the front of their cylinder. Notice I said most. I have a RSS5W that letters as a single cylinder gun that has the last 3 digits of it's serial number scratched on the front of the cylinder. That being said when Ruger first started making the convertibles they stamped the last 3 number of the serial number on the front of the extra cylinders. That means the .22 mag. cylinders were stamped on the RSS4X, RSS5X, and the RSS9X,, and the .22 L.R. cylinder was stamped on the RSSMX, as that was the extra cylinder. This went on to about serial number 360361 for the RSS4X, 5X, and 9X. and serial number 360532 for the RSSMX. After that Ruger starting scratching the last 3 number of the serial number on the front of the extra cylinders. Shortly after that Ruger started scratching both cylinders with the last 3 numbers with the serial number and continues to this day.
As a general rule and probably 99.999% of the time if you have a RSS,4,5,9,M with a cylinder that has the last 3 numbers of the serial number scratched on the front of it, then most likely it is a convertible, and I would say 99.9999%. :? :? :? One other thing to add. Once Ruger started the convertibles all Old Model Single-Sixes with 4 5/8, and 9 1/2 in. bls. were convertibles