That is not a serial number (that gun does not have one) it is a model number. Your shotgun is a High Standard Flite King- good guns, by the way. They sell used for about $130
"Shotgun" means the passenger side.
I am guessing you have a model 870. It is a edition of the 870 shotgun
This is a double barrel, over/under shotgun.
If you call out shotgun it means you get the front passanger seat in the vehicle.
The number 1400 refers to the Model number of your Winchester shotgun.
a shotgun
Improved cylinder
Unknown.
Go here for a good explanation - http://hunting.about.com/od/shotgun/l/aastshotgunfaqa.htm
Degree of choke
A fowling piece is a type of shotgun.