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.
Try the blue book of gun values base on condition
45 yrs old
50-250 usd
Need a detailed description of finish, condition, box, papers, all features and markings.
Your browning light twelve was made in 1972 as the number prefix suggests.
Your browning light weight auto-5 shotgun was made by FN of belguim for browning in 1972.
1972. Can't address value a detailed description of all markings, finish, barrel length, condition, accessories, box, papers, etc..
A Browning light weight auto-5 shotgun in unfired condition,and made in the year 1972(72G)prefix to your serial number will bring 775 dollars for a shotgun with a plain barrel,or 1,150 dollars for a vent rib barrel model.Yours is often called the flat knob model(which was made from 1967-1976).
The 72G prefix dates your browning auto-5 light weight shotgun to 1972.
100-700 usd
1972; 100-1000 usd
About $400 in good condition