Howdy Sean - Here's some additional info...
There's been a lot made over the years of the iron-mounted Henry rifle. Some have even postulated it as the forerunner to the iron mounted plains rifle.
My considered opinion is that the iron mounted Henry's, which first came out in 1830, were in response to the iron mounted rifles that were already being used in the west i.e. Southern Mtn guns, Hawkens, and the English guns, rather than a precursor. Henry was a smart businessman and IMO more of a responder than an innovator (i.e his English model was a "copy" of the imported English trade rifles.)
About 1831 he also started selling half-stock rifles, which based on the companies records of sales of other models were most likely flint! (While a generally a good responder to the market Henry doesn't appear to have offered percussion models until around 1840.) By mid-1836 he was shipping these half-stocks on a regular basis. Sure would be nice to see what these looked like.
However, but it (iron mounted rifles) appears to have been something rare of a special order that was short lived. Hanson only found a handful of these orders in existing AFC records.
While Hanson states the iron mounted Henry's were only built from 1830-1834 other documentation says otherwise. On both the 1836 and 1837 rendezvous trade lists from the UMO (sent to rendezvous from Ft. Pierre rather than St Louis - Hanson did not include these records in his research, IMO a MAJOR faux paux that scews the facts), iron/steel mounted Henry rifles appear, these may have course been old stock.
But, in 1844 and again in 1846, T. J. Albright of St. Louis, ordered blued, iron mounted rifles from Henry. (page 51, Firearms of the American West: 1803-1865). Some of those rifles were half-stocks with walnut stocks and back action percussion locks.
No one really knows what these looked like, but they were likely similar to the guns they were already making like the Lancaster or English patterns.
What did the iron mounted Henry's look like - no one will know until an actual one is found or a period description pops up, but I've got a "feeling" that these may have been the progenitor to the Scroll Model English which appeared in 1834?
As to barrle lengths, all the major students (Serven, Barsotti, Hanson, et al) of the Hawken state the majority of them had barrel in the 38-40" lenth, but exceptions do exist. Two of the earliest Hawken in existence (see Baird, Hawken Rifles, the Mtn Man's Choice), have shorter barrels. One an early half-stock has a 33 5/16" barrel and the other a full stock has a 36" barrel. While IMO not as common as the longer barrels, shorter barrels are not completely unknown during the 1820-1830 period. The Harper's Ferry M1803 half-stock of course had originally a 33" barrel and the later version had a 36" barrel.
That's all for now - got some hides to stretch..
BTW - Sean - give me a shout some time - been a while since we palavered. Finally catching up on orders and am able to breath!