May not be marked, depending on age.
"P" prefix indicates 1953
Harrington & Richardson used the Prefix AT in 1983.
How old is my Topper 88 HARRINGTON AND RICHARDSON GUN serial number
Harrington & Richardson did not make a Model 1900. Hopkins and Allen did.
1971
Serail number and features
1980
50-250 usd
No way to answer without the serial number.
Your harrington and richardson model 900 was made between the years 1962-1973.
I purchased a new Harrington & Richardson 12 gauge model 088 from K-Mart in 1980.
"AY" serial prefix = 1982.