If your over under shotgun was made prior to 1964,then the model of your Browning shotgun should be named the superposed.
Your Browning superposed,over under 20 gauge shotgun was made in the year 1951;with the serial number that you have provided.
Best left to a gunsmith.
40891 s5
A few hundred to a few thousand.
100-1000 or more depending on specifics
You will have to contact Browning and order a history letter.
800-1100 USD or so
John Moses Browning. The Browning Superposed was the first practicle over, under shotgun
Is it a pump, bolt, over/under or semi-auto?
100-10000 USD depending on specifics.
Browning offers history letters for @$40 USD