It was because folks were asking for the other caliber for their guns. In 1959, the Magnum Single-Sixes were produced, and later on, when they decided to make just one model, they made convertibles. Any fixed sighted Single-Six with a serial number before 150,000 did not have the mag cylinder. The number places it in the production era after the magnum cylinders were being added to the Single-Sixes. So, as you already know, your gun is a FIXED sighted gun, and that makes it a 'Old Model Single-Six Convertible.' It's not marked 'magnum' or it would have a different serial number block. MUCH better than any other resource for us.
You can join RENE, and Chad (chet15) has a Reference Guide that is INVALUABLE to collectors. As FM alludes to, we use the RENE Reference Guide for information.
