Try e-gunparts.com
I can't find any reference to a Stevens mod 320 however if it is similar to their mod 124 (12 ga, bolt action, 3 shot box magazine, 1950's manufacture) then you will not find an extended magazine for this gun. Stevens never made a 5 shot magazine for this gun and no after market companies would either. A 12 ga pump with extended tube magazine is a better solution.
http://www.e-gunparts.com/
Your Stevens model 77F was produced from 1955-1971.
i have one that is a model 77e
Stevens Model 77C comes in 16 Gauge . Made in the 60 -70s
Your Stevens model 77C was made from 1955-1971.
It's a Stevens Model 520A.
You can find what you need at webdept@gunpartscorp.com .
Try this link: http://www.gun-parts.com/savagestocks/
That is not a model number, but is likely the serial number. Unless you have a Model 620 US marked Trench Gun, Stevens pump shotguns have little collectors value.
It is a 12 GA pump action shotgun.
July 10, 1902 Patent application was filed on a pump action shotgun that would become the Stevens Model 520.