A bit more about this Alexander Henry:
There were only 88-90 total hammerless single shot actions (and not all even conform to the several different Patent designs or specifications) produced by Henry, and of those, perhaps up to 20, were not finished into complete rifles. It appears from the records that 47 were made as sporting rifles, the balance were made as military pattern for government trials or as gifts by the British and/or Henry to other interested governments or influential individuals.
Of the ~88 Alexander Henry hammerless rifles produced, there are only 14 known examples, this Action #18 is one of those known 14.
It is based on the 1870 Patent, has a thru bolt to secure the buttstock, internal side hammer, and a lever actuated “beam” that actuates the breech block. The firing pin is not bushed, but rather has a rear retention plate similar to a Sharps rifle.
As can be seen in the last picture, the top cover/loading trough is removable. The actual entire action lockwork, trigger, and lever are mounted on a lower action plate and are removed from the bottom. The buttstock and forend do not need to be dismounted in order to remove the action plate.

A few more pictures

Top cover plate removed, note the internal side hammer: