Value is too difficult to be accurate without seeing the gun. Condition which includes the amount of original finish and original configuration will determine value. Auto 5's with poly-chokes, compensators and recoil pads (other than Mag 12's) will reduce value. Near mint guns and original box will command an extra premium. Recommend taking it to a gun shop for appraisal.
A few hundred to a few thousand.
The A5 20 ga. was made in two models: Light 20, and Magnum 20. The Light twenty, sometimes just reffered to as a "Browning Twenty" was made from 1958-1976 in Belgium. The receivers are marked "Light Twenty" The Magnum version has a 3" chamber and was made in Belgium from 1967-1976. The receiver on this gun will say "Magnum Twenty" Other differences are the weight of the models
50-500 USD or so
300-600 USD.
Browning.
100-400 USD
Thats a a5 magnum twenty made around 1970 value 1400 in mint condition
@ 400 USD
Impossible to answer without a detalied description of all features and markings.
300 USD
75-575 usd
Weight.