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.
How old US a Stevens savage model 530a 16 gauge
about £120
The stevens model 9478 shotgun was made between 1978-1985.
Just another model in the Stevens line.
$75 to $125
Whats it worth?
Your Stevens model 77C was made from 1955-1971.
Stevens 947y This shotgun is a Savage 940,940Y,or 940B
A Savage Stevens model 94 shotgun was made from 1926-1945,there were 934,000 made during this time frame.
$1899
200,375
$17 in good condition