There is very limited information on these revolvers, and the serial number you posted does not track with what information I have. The true serial number on many of these is stamped on the inside of the top of the frame, and the cylinder must be removed to read it.
Harrington & Richardson used the Prefix AT in 1983.
1973
How old is my Topper 88 HARRINGTON AND RICHARDSON GUN serial number
"P" prefix indicates 1953
serial number suggests a single barrel, hinged frame Model 1908/08 shotgun - if so - 1920-25.
1987
May not be marked, depending on age.
1971
Serail number and features
1980
50-100 USD
Try Cornell Publications. They have a few books on the company