Your harrington and richardson model 900 was made between the years 1962-1973.
Blue Book of Gun Values
Have you tried a web search? Local Library? Do a web search on Harrington and Richardson Books and you will find some sources.
You will have to measure the inside diameter of the barrel to find out if it is not marked.
Try e-gunparts.com
You will have to contact H&R 1871 Inc. to find out.
Try the following link. I saw it there. www.gungarage.com/gunmanuals.htm
gun parts corp has them on their website.
Does not exist. You might find a magazine at egunparts.
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.
The H&R topper model 490 JR was made from 1961-1983.I hope that you find this informative.
If you took it apart without an owner's manual and now can't get it back together, find a gunsmith.
I find no Harrington & Richardson S/S shotgun manufactured since 1885.