I Just Purchased a model 66-4 and mine says, Made in USA Smith & Wesson Sprinfield, Mass
The frame size of a Smith and Wesson model 66 is K.
Only Smith and Wesson knows and they aren't telling.
With the serial number that you have supplied,your Smith and Wesson model 66-1 was made in the year 1981.
1971
@ 1971
1975
It was made in 1987
The Smith and Wesson model 66-1 was made from 1977-1981.
1985
About 1988 or 1989.
Impossible to answer without the serial number.