Thanks for posting this! We don't see very many Richmond guns posted.
As you already alluded to, aside from the lock everything else is consistent with the Model 1855 except for a handful of things. The early Richmond guns also used left over HF parts, so it becomes kind of hard to be sure of anything. That said, the rear sight itself is a later pattern although the barrel has the 1855 cut out. The screw holding the sear on the lock looks like it has a rounded head instead of the expected flat top, although I may just be seeing things, and the main spring appears to be a later pattern as well since it doesn't fit very well. None of these are big deals per se, but it does add to the difficulty in knowing who assembled the parts into the present rifle-musket, and when it was done.
Richmond locks have been assembled onto Model 1855's by people wanting a bigger return so it is certainly a guessing game. I recently spent a fair amount of time studying the online offerings of the bigger dealers for my own amusement and it was surprising to me how common it was for guns to have incorrect parts being sold for very large dollar amounts.
I have a very similar rifle-musket with an 1861 dated Richmond lock, and like you, I bought it for about the price of the lock. I assume mine is just a post-war assemblage of parts. However, it fills the hole in my collection at a fraction of the price and I am too wary of how easy it would be to fake one of these to lay out serious money for one.
The star marking on the butt plate is intriguing. I'm sure others will be along that can shed some light on that. It has been a long time since I read the book on these and I only have a vague memory of where I have seen this before.
If you would like some replacement screws for the sear and sear spring send me a pm. I have a bag full of them and would be happy to send them to you.