Ruger is usually pretty good with info. In this case, I think you got some mixed info. The MFG date of 1964 you received would be for a
Super Single-Six with SN 503623 (no D).
The
Single-Six didn't reach SN 503623 until 1968. In 1964 Ruger introduced the New
Super Single-Six and assigned SN block 500,000. 4 years later the Single-Six SN's finally caught up to the 500,000 block of numbers assigned to the Super Single-Six, and about 50K Single-Six guns were produced with the same SNs as the Supers. By this time the Government had mandated that each firearm at each individual manufacturer had to have it's own unique SN, not just a different model designation. To comply with Gov. Regs. the Single-Six guns with "Duplicate" SNs were stamped with a "D" and the SNs were jumped to the 800,000 block, soon after the prefix numbers started to appear.
Thats pretty much how I understand it.