Try "googling" Printable/Down-loadable Owners Manual for Harrington and Richardson Model 88. I did this for an NEF Lil Pardner 12 ga single barrel and an older Mossberg 12 ga Model 500 and it took me to a sight where I printed out both of them. J and G Sales has quite a few printable manuals advertised on their web site but normally they are for item which they normally sell.
Harrington & Richardson did not make a Model 1900. Hopkins and Allen did.
Harrington & Richardson used the Prefix AT in 1983.
I purchased a new Harrington & Richardson 12 gauge model 088 from K-Mart in 1980.
Your harrington and richardson model 900 was made between the years 1962-1973.
If you took it apart without an owner's manual and now can't get it back together, find a gunsmith.
25-100
10-110 usd
1871
36GA
2013
between 1949 and 1952