That number was likely issued in 1970, but the gun probably shipped in 1971. But only a letter can tell you with certainty.I bought a Model 10-6
The serial number is D265470
A Model 10-6 has the heavy barrel by definition. The equivalent gun, made in the same period, with the "standard" or tapered barrel was the Model 10-5.with a heavy 4" pinned barrel
But only a letter can tell you with certainty.
The end of the run??? My 10-6 4" pinned HB is 38 special only ser # D8717**
A letter is a little steep for me
"Enclose check or postal money order payable to "Smith & Wesson Historical Foundation" at the rate of $75 (U.S. Funds) for each "Letter" requested."
Yes, with D 871xxx you'd be in 1976 and the change to the 10-8 happened in 1977, but what I don't know is whether that still happened within the D prefix (before D 999999) or after the D started floating to the right.