Fulton-
Take your request to nitroexpress.com
There you will find the highest standard of accountable information on the web in regards to double rifles.
I agree with the above notes that what you have there is a Black Powder Express rifle.
The lever under the trigger guard rotates to the right of the gun to get the action open.
If it's stuck, don't force it.
The rifle seems to have been refinished at least once, and the work was not nearly to the standard of the rifle's maker.
It also appears as if the stock has been cut and re-lengthened at some time.
If you can get it running properly - and I warn you, there are very few men here in the US who are qualified to work on that gun (JJ Perodeau of Champlin Arms is at the top of the heap, google him) and if you allow anyone besides an absolute top-notch professional Double-Rifle mechanic or Gunbuilder work on it you stand to loose a lot of value, and likely the possibility of the successful function of that very nice H&H rifle.
Hammerless Back-action Sidelock Jones Underlever guns and rifles are relatively rare, that lever is somewhat 'spring loaded' in that it's not only unbolting the barrels, but it's also cocking both of the locks.
Be careful, learn more about what you have, and listen closely to what you're hearing about that nice old rifle.
--Tinker
