Fine work, you know your stuff. Lewis and Clark carried a 1803 Harpers Ferry 50 cal half stock flintlock. Your build looks like the transition rifle, half stock for horseback, a forerunner of the Hawken.
I am looking for the documentation, but recently I read that the 1803 Harper's Ferry Rifles were delivered about 4 months after the Corps of Discovery shipped out.
The gun they carried that really gets my interest piqued is the air rifle. They were able to get multiple shots off before pumping up the reservoir. Coooool.
I just found out something. I was in Asheville NC today and as my custom I went to a used book store to brouse around. I found a book titled Lewis and Clark Across the Divide by the Missouri Historical Society and Smithsonian Books. The book is a companion catalog of items carried in the Lewis and Clark expedition collected from several museums. The large nice book was $8 so I bought it and as soon as I got home I looked up weapons.It seems as if my assumptions about the party carrying Harpers Ferry 1803 half stocks is wrong. The book states that time time frame is wrong for the HF 1803's to be used. It seems that the party got fifteen Model 1792's that were stored at the HF arsenal and had them fitted with new locks. They also had duplicate locks made to carry along with the reconditioned rifles. The Model 1792's were originally made by private gunmakers on contract with the US government.
All this fits with my previous research, guns from HF and same locks and parts. Ya learn something new every day. Sure glad you brought this to my attention JW.
By the way the book has all kinds of other way cool info about the LC party including published photos of original journals and lists of trade goods and such.