The 20 indicates that your gun was made in 1980. Marlin did not checker the stocks on the 39a at that time period.
The year of manufacture for 10 = 1990.
The Limited had a 24" bbl., straight stock, no checkering, and a smaller forearm . Two versions - 1/2 round, 1/2 octagon, and full octagon.
The Marlin Model 3000 was the chain-store version of the Model 336. They added a gold trigger, gold bead sight and have no checkering.
Checkering refers to the diamond patterned designs etched in the stock of some shotguns and rifles.
Bolt action single shot .22 made from 1966-79 and now discontinued. Price range $40-70. Same as the Marlin 101, except with walnut-finished wood stock. Later 1970's productions w/ hot-iron wood stamped pistol-grip to simulate checkering. Plain forearm.
go to www.auctionarms.com and check the value of similar guns.
Checkering is cutting or carving very thin parallel lines into the wood. This is done with a very fine chisel blade, called a checkering tool.
As far as I know only the Model 64 Deerhunter had factory checkering. Sometimes this is called the Deluxe M 64 . Hope this helps.
The rifle is worth $1100.00 at 100% condition. $350.00 at 60% condition. This price is for the diamond weave checkering. The basket weave checkering is worth about $200.00 less money. If it is the carbine (barrel band and no checkering) add a couple of hundred dollars.
1 marlin several marlin.
The 120 is a 1300 with a hardwood stock and no checkering.