Very collectible revolver - I think you have the serial number incorrect - supposed to be only around 1000 made - in Fair condition $300 to Excellent $800. Holster ?????????
0-100 USD or so
Your harrington and richardson model 900 was made between the years 1962-1973.
25-75 USD or so
No such maker. However, there is a firm called Harrington and Richardson and they did make pistols in 38.
50-100 USD
1971
10-100 USD or so
Blue Book of Gun Values will help.
"P" prefix indicates 1953
Trademark of Harrington & Richardson Arms company
50-125 USD
50-125 USD or so