That is why your gun has the 'D' stamped as part of the serial number. After they had serial numbered a few thousand, someone at the factory caught the mistake and had a 'D' put on the serial number. This was in 1968 and the factory forgot that in 1965 they had serial numbered Super Single Sixes in that range.
When the Single Six got to 499,999 it went to 500,000. The super Single Six was assigned the serial number range 400,000 only 5 were made with that serial number 400,001-400,005 and the 500,000 range. And the Single-Six like yours and the Super Single-Six. They did he same thing with the Single-Six and the Light Weight Single-Sixs. They did the same thing with the Standard Autos and the MK-I. Ruger has the policy of setting aside serial number blocks for different model guns. The Super Single-Six that has the same serial number that your gun has, minus the 'D' was made in 1965. Your gun was probably made around 1968, a call to Ruger will give you a shipping date. It's a Single-Six in the Super Single-Six serial number range.
