What year was my gun made?

Question
I bought a .410 off of a friend for very cheap, I had it ran through the system to make sure it was all clean and not stolen. I am trying to find out as much information on it as possible, starting with the year it was made. I can't find anything on it though. Could you help me find as much information as possible on it, please?  
This is all the writing it has on it,

New England Firearms,
Manufactured in Gardner. MASS. USA
U.S. Pat NO. 3988848
Serial number: NH 525813

Pardner Model: SBI
.410 GA. 3" full
There is the letter P in one spot with a circle around it.

I would greatly aprreciate it if you could find anyting out. Thank you.

Answer
New England Firearms came about after H&R went bankrupt in late 1980s.
Its an inexpensive shotgun.  Its still made.
http://www.hr1871.com/about/default.asp
There no public data base for dating by serial number as far as I know.
You try asking  H&R forum.
http://www.go2gbo.com/forums/index.php/board,132.0.html?PHPSESSID=vgcu3niaup6rv9...

The patient for transfer bar safety.

Full is choke constriction.

(p) is proof mark that it was pressure tested.

The factory in Gardener was closed after Remington bought the company.