Looks like an Evans 'New Model' Sporting Rifle. They had the standard 'Evans Repeating Rifle. Mechanic Falls ME. USA. + Pat dates' on the bbl.
Cal 44 Evans N/M (New Model). Other models were cal 44 Evans Short.
The Sporting Rifles were made w/ octagon bbl and available engraved and checkered stocks.
Only the 'New Model' used a sliding dust cover over the ejection port.

(There was another 'Evans Sporting Rifle' and marked just as such on the bbl. But these were rifles made from left over parts and sold when the company went banckrupt. Most of these 'Sporting Rifle' marked guns use left over & sometimes cut down 30" bbl that were originally used on the Evans Military Musket.)

Auction says the rifle has old English proof marks and the name T H Brown stamped by the trigger (best I can determine). A ser# of '7'
I don't know if the TH Brown, Confederate soldier turned gunsmith in 1870 they speak of is the same one who's name is punched into this rifle w/ English proofs,,but who knows.
Maybe an English custom & upgrade of an Evans w/a retailers mark ( T.H. Brown)on it.

Nice rifle,,crazy 4 row helix magazine tube forming the center of the butt stock.
I haven't seen one of those around in quite a while.