The overstamp means that the frame was marked for a 3in, heavy barrel version. It was shipped as a 2in Model 36 no dash, with a 0 overstamp.
The 36 was built from 1957 until 1988, when the 36-2 was introduced.
The 36-1 was started in 1967 and until 1988 when the 36-3 started.
It had to be built prior to 1966, because that year they when to the modern style thumbpiece.
Please check the serial number, The Standard Catalog of Smith & Wesson doesn't show any number in that range.