Browning Lightning 20 o/u with vent rib

Question
Weapon has serial number of 20518.  I am wondering when weapon was manufactured and what its value might be.  Has only been fired about 100 times.  Thanks you in advance

Answer
I will have to get back with you later this week. I am out of town right now and will have to check my literature when I return. I suspect it is made prior to 1960, but will have a better answer for you soon. You might consider going onto the Browning.com website they have an area which has the dates of manufacture, otherwise I will get back with you.