Check the barrel and receiver. Sn's were not required before 1968
Dating a Stevens 12 gauge model number g20a and serial 12202 is best done by the name on the gun. Where only Stevens appears the gun is likely made between 1940-1991.
4
No way of knowing without a serial number /model number to identify what type of .410 gauge stevens you have.
model 77 does not have a serial number,they were made before serial numbers were required
Your stevens model 9478 was made by savage/stevens between the years 1978-1985.
If it was manufactured before 1968 it probably doesn't have one.
The Stevens Savage Model 124 12 gauge shotgun was made from 1949 to 1954. The serial number will signify the actual year of manufacture.
Look on the top of the receiver with the gun disassembled. You should find it there.
The serial number range for the Stevens model 520-30 military shotguns is from 30,000-70,000.As to the year of manufacture you will probably have to contact Savage/Stevens directly.
Your stevens model 9478 shotgun was made during the years 1978-1985.while serial numbers were on the shotguns I do not believe that serial number records were kept as to the year of production.
Stevens, sold at Montgomery Wards.