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Trademark of Harrington & Richardson Arms company
0-100 USD or so
$100 to $400 US depending on the condition.
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
Harrington and Richardson, an American gun company that made a LOT of inexpensive revolvers. But you need to ask an actual question.