More information needed.
Harrington & Richardson made a top break 7 shot 22 rf called a premier.
Sorry- not enough info to answer. The 7 shot revolvers were early guns- I have one made about 1900.
Your harrington and richardson model 900 was made between the years 1962-1973.
25-150 usd
not sure, but I thinks it's a 22 cal...............
Best left to a gunsmith
$200
$100
50-100 USD
$40-$150 depending on the condition.
50-100 or so
"P" prefix indicates 1953