Bill Goforth has an excellent book on the H&R shotguns. You can do an internet search to find a copy.
Blue Book of Gun Values
I find no Harrington & Richardson S/S shotgun manufactured since 1885.
There are some published serial number lists.
this shotgun was made by Harrington & Richardson and is there model 120.www.e-gunparts.com sells some parts for this gun. searching under Harrington & Richardson model 120 will yield more results for you. good luck.
Unlikely that detailed sn data has been published.
Please list all the markings on the shotgun. Harrington & Richardson were producing hammerLESS double shotguns as far back as 1882. sales@countrygunsmith.net
If you took it apart without an owner's manual and now can't get it back together, find a gunsmith.
Your harrington and richardson model 900 was made between the years 1962-1973.
An inexpensive shotgun made by Harrington and Richardson in the late 19th or early 20th centruy. . Value 25-150. Best thing to do is hang it on a wall.
Made through the late 1940's - mid 1950's. Value under $100.
Cornell Publications
Try e-gunparts.com