Check w/ Marlin through their website, customer service.
Your Glenfield model 30A was made in the year 1979.
With the serial number that you have provided,your Glenfield model 30A was made in the year 1977.
1976
With the serial number that you have supplied,your Marlin(Glenfield) rifle was made in the year 1980.
Double check your serial number. sales@countrygunsmith.net
Your Marlin model 30A was made in the year 1978,with the serial number that you have provided.
1978
I would check the lower tang,under the lever.
1976. www.countrygunsmith.net
Your serial number dates your rifle to the year 1982.
1980.
Subtract the first 2 numbers from 100 = 1978.