The Iver Johnson web site has some info
That should be 1975
S prefix is 1956.
Serial numbers were not required before 1968
Serial numbers were not required until 1968
Harrington and Richardson used serial numbers that started with AM in 1974. The company had similar shotguns being built from 1974 to 1981.
The prefix letter shows the year of manufacture. 1956 is letter S
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
50-100
1987