The precise YOM is going to be hard to determine. It sounds like you know it was made between 1971 & 1979. With the low serial number, I'm guessing you are right that it is an early one.
There were 12,089 of the 120 - 28" barrels produced from the 48,6XX total 120 magnums made.
I'm just not sure of the value. These sell in the 200 - 250 range in this part of the country. In a region where trapshooting is very popular, they could well bring more than that. SW