There is no way to age your browning auto-5 shotgun without a serial number.As to the value,this would be 450 dollars for a shotgun in the condition that you describe.I can say that the Browning auto-5 standard weight shotgun in 16gauge was made from 1946-1964.
A Browning auto-5 standard weight in 80% condition will bring between 325-475 dollars depending on which type of barrel the shotgun has.plain barrel(325 dollars)matted rib solid(375 dollars)and vent rib(475 dollars).
If you have a standard barrel then 525 dollars,if you have a vent-rib barrel then 735 dollars.You made add 40% to the above stated prices because you have a 20 gauge model.
50 USD
Try gunbroker.com in the barrel section.
100-1000 USD or more depending on specifics.
The barrel will be marked "Made in Belgium"
It will be marked on the receiver/barrel.
Your Belgium made Browning standard weight 16 gauge shotgun,which was made in 1955 will bring between 325-375 dollars in the condition that you describe.If it has a vent rib barrel then add 50-75 dollars to the above listed prices.
Which model is this? Simply check the barrel. It should say Made in Belgium if it was.
Your browning auto-5 standard 16gauge shotgun was made by FN of belguim for browning in 1953.
Your browning auto-5 standard weight 16ga shotgun was made for browning by FN of belguim in 1927.A fine browning auto-5 with this age and in 80-90% finish will bring between in the neighborhood of 600-700 dollars.You can add a little more if you have a solid rib,or vent-rib barrel with the shotgun.
50-500 usd