Some resources on Henry and other western fur trade rifles:
1) Firearms of the American West: 1803-1865 by Garavaglia and Worman - IMO a most valuable resource due to it's use of primary documentation especially orders, It has info on the Henry's scattered through out.
2) Gunsmoke and Saddle Leather by Charles Worman
3) The Hawken Rifle: It's Place in History by Charles Hanson, Sr - not only has a chapter on the eastern trade rifles, but also has a chapter of period adverts from St Louis which illustrate how varied the goods being offered were
4) The Fur Trade Rifle Sketchbook by Charles Hanson, Jr.
5) Firearms, Traps , and Tools of the Mtn Men and Guns on the Early Frontiers by Carl P Russell
6) Kaufman's and Dillin's books on the Pa & Ky rifles
Most of the sources are a bit dated so read them all with a grain of salt so to speak and cross reference as much as possible.
Sean has also graciously sent me copies of various magazine articles he has found. He also has the contact info for Jim Gordon, who is offering a set of high quality, full-color books chock full of firearms of the fur trade - only kicker the price - about $300.00
From about 1780, the English were shipping trade rifles to America which were their version of the American Long Rifle - this pattern is what Henry from 1826 on, offered as their English model and after 1834 as their Old English model. After 1830 Henry was the main supplier of both English and American/Lancaster styles of rifles to American Fur and it's later permutations under various names as run by the Choteaus.
re: Water proof locks:
1) In 1830 Ramsey Crooks of AMFco ordered rifles from Henry and in his letter stated " ....I think the Locks should be waterproof..."
2) AMFco order Oct 1832: 10 rifles.....bll. 3f 4, 32cal (ed: 32 to the lb .526" ball size), 5In watpr locks maple stock....
oval cheek piece, long bacstrp, bll browned (an 1833 order also calls for browned barrels)....
10 rifles as above 3f 4, cal 28( ed: .550" ball size)
So we have at least 20 trade rifles ordered with water proof pan locks....
re: 5 inch locks on Scroll Guard aka New English Henry's
In 1843 the Ewing brothers ordered 270 Old English and Scroll guard rifles, the latter to...
"put on the same quality Lock which you put on the Eng Pat Rifle only not so large - say 5 inches...."
The Iron Mounted Henry which is illustrated in several of the above named books was not built by Henry as Larry noted above, it has a Henry barrel and IIRC lock, but was built by a North Carolina maker and is so signed. Henry sold barrels and locks as well as completed guns.
I Photoshopped the pic of Larry's 6" lock for a bit more detail....