Cornell Publications
Manufactured by Harrington & Richardson very near that patent date.
Your harrington and richardson model 900 was made between the years 1962-1973.
it was with you mom your mom made it here bad spot
Try e-gunparts.com
I find no Harrington & Richardson S/S shotgun manufactured since 1885.
Have you tried a web search? Local Library? Do a web search on Harrington and Richardson Books and you will find some sources.
Blue Book of Gun Values
http://www.e-gunparts.com/productschem.asp?chrMasterModel=1560z949
Try Gun Parts Corp
There are some published serial number lists.
Try e-gunparts.com
You will have to measure the inside diameter of the barrel to find out if it is not marked.