There is no fixed or standard length. There are serial numbers of different lengths for different purposes.
Sn is incomplete
Standard Catalog of Smith and Wesson, 3rd, ed.
Standard Catalog of Smith and Wesson by Supica and Nahas
I don't know if all High Standard A-100 rifles (semi-auto, .22LR caliber) had serial numbers. Gun companies didn't have to use serial numbers until 1968, and a lot of inexpensive .22 rifles and shotguns didn't have any serial numbers back then.
Your serial number indicates that your Hi-Standard model 101 Dura-matic was made in the year 1969.The serial numbers for that year are from 2,030,405-2,172,356.
The "Standard Catalog of Smith and Wesson" by Jim Supica
Standard Catalog of Smith and Wesson, 3rd ed.
Standard Catalog of Smith and Wesson, 3rd, ed.
Places to look; barrel, frame, receiver The K-1211 sereis was produced before High Standard put the serial numbers on their shotguns except for a small number that were esported.
SN does not have enough numbers.
It is a Standard weight 12 ga made in 1960.