When Ruger introduced the .22WRM Single-Six in 1959, they made a few, uncataloged, convertibles by adding a LR cylinder - which got it's first advertizement in 1961 (in the May AR) made it to the catalog in 1963, IIRC.
The WRM-based version started at about serial # 319,500, until approx #340,000.
The convertibles were then (1963) changed from .22 WRM's (marked on the frame) w/LR extra cylinders to standard .22LR's (no WRM frame mark) w/WRM extra cylinders.
The earliest standard SS convertibles were introduced into Ruger's lineup at approx serial # 169,000; the post 1963 models started at #340,000, running up to the 700,000 range before the 1969 serial nember prefixing system went in (20-xxx, 21-xxx).
There were many different collectible changes over those years - including the adjustable sight Super SS - right up to the switch to the transfer-bar New Models in 1973.
Some Single-Six's, mostly those with a serial # below 150,000, were fitted with an extra WRM cylinder by someone - which gave accuracy problems related to bore size.
Ruger refused to do the conversion, on revolvers sent in for work, below that serial #, but did so on revolvers with higher serial numbers - so one could presume Ruger made a change to the bore size about then.
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