I'll agree with Louie again, as he's right most all the time! ;-)
As far as when the work was done, to help date your gun, no way really to tell definitively.
However, the lock isn't a Dickert era lock, in fact is an original percussion lock, say 1840-ish, and I think the drum was new along with the lock. So your gun could have been made around then, or a good bit after that as well.
The pic showing the under lug at the breech end looks to be the original Dickert type lug, just moved up the barrel by some inches. If you measure the distance between the old lug position and the new one, that's the amount that was cut off from the rear of the barrel.
Here's another Dickert signature on a cut down barrel.
Any way you look at it, it's a nice gun to have passed down!
John