OK, It's becoming clear as mud. It's a 5 1/2 incher.
So a L.R. cylinder would likely be correct.
Well, it would at minimum have a LR cyl. It may very well be a convertible that is also missing the WMR cyl.
At that point, the single cylinder guns were pretty scarce. A letter would be the only way to know for sure.
In the mid-1960s period, your gun would be either a RSS5X (convertible) or RSS5W (single LR cyl). They started using the W, which in the XR3 days stood for Wood grips, to denote single cylinder guns in that early XR3-RED period. I have RSS5Ws in the 38xxxx and 39xxxx ranges and also two in the 82xxxx range. But there should also be a few in the 4xxxxx range.
Inconsistency was the only consistent thing about Ruger's catalog numbers and serial numbering.