What they said. If it's in the condition you describe and functions properly it's tagged/priced pretty low (on the National Average scale.)
My suggestions for checking the workings:
1. Cock the hammer once for each cylinder chamber - make sure the cylinder moves 9 times and the fore-aft end shake is not excessive and the wobble - left to right movement is tight upon lock-up. NOTE: do not dryfire this gun - hold hammer spur with thumb, pull trigger and gently lower the hammer - it will go to rebound position.
2. Remove the cylinder by pulling the cylinder pin. Then try the trigger action and hammer movement in double action and single action a few times. SA should be around a 4# pull, DA may be as high as 12# - but both should feel solid and have a clean let-off. While testing the SA trigger, with hammer cocked - push forward on the hammer to test lock up on sear - if the hammer falls while doing this - somebody has messed with the sear/hammer notch or it's broken - neither condition is desireable.
The marking on the cylinder PAT. 1904 is part of what the original stamp was.
It looks to me that H&R used the same PAT. # stamp for years - gradully wearing down the numbers until only a vestige was left to make the imprint. I can see this degradation occuring when comparing cylinders made in successive years. Eventually H&R stopped using the stamp - probably when it no longer made any imprint at all.
The mono-grip is made of an aramid Nylon type plastic, used almost exclusively by H&R during a after the WW2 era. Check the pic. below of a 4" bbl example.