bigred0383:
We are working from the same reference, so let's narrow it down.
1D70xxx dates to 1979-1980. Serial range for those years was 1D45001 to 1D99999. The 1Dxxxxx serial range was used for the Model 64 & 65. (See Page 399 SCSW).
The Model 64-3 was introduced in 1977 and continued in production until 1988. The pinned barrel was deleted in 1982. (See Page 225 SCSW).
So if your gun is a Model 64-3 with pinned barrel then it is correct for a gun produced from 1977 to 1982, which means it is correct for the period 1979-1980.
There isn't anything special about the 1D serial range, it was just a case of S&W running out of Dxxxxx serial numbers. Again looking at page 399 of the SCSW, you will see the progression of serial numbers for the fixes sight K frames, starting with C1 in 1948 & running up to C999999 in 1967. They then moved up a letter to D1 in 1968 & continued through to D999999 in 1977.
It appears that In 1977 for some reason instead of using an E1 prefix they split production with a 1Dxxxxx for stainless guns and 2Dxxxxx for blued guns. In 1981 they remerged the serial ranges as the 1D range had been used up. This is probably why there is no 3Dxxxxx serial range, it was reserved for the stainless guns but not used as the stainless serial range merged back into the blued serial range, which by 1980 had already worked its way through the 2Dxxxxx, 4Dxxxxx, 5Dxxxxx & 6Dxxxxx serial ranges and was starting on the 7Dxxxxx range.
As for the C1, C20 & C70 stampings, they are all assembly numbers, used to track parts in the factory. If you take the grips off you may find more on the flats of the grip frame. None of them are relevant once the gun is completed.
TL,DR: Your gun is not a special run gun, it's a normal production run firearm from 1979 or 1980. Enjoy shooting it.