Glenfield,which was made by Marlin Firearms Company made a bolt action .410ga shotgun from 1956-1965.This was the marlin model 59 which is the same shotgun as your Glenfield model 60.
Go to Cheaperthandirt.com
Your serial number indicates that your Glenfield model 20 bolt action was made in the year 1980.
Bolt Action 12 gauge mod 50.You can get one in geat condition for about $80 to $100.
A utility bolt action 12 gauge will bring from $50 to $125, depending on condition, at most auctions.
$100
1932-1957
write to marlin with any and all writngs/markings on reciever/barrel and serial numbers
Model 58 is an automatic not a bolt action
1978.
$25 to $125, depending on model and CONDITION.
THIS Glenfield MOD 50 BOLT ACTION SHOTGUN WAS MANUFACTURED 1966-1972. I HAVE ONE AND ALSO CANNOT LOCATE THE SERIAL NUMBER.
About 100 USD.