Dennis,
That style of breech plug was likely developed relatively early in percussion cap period. I believe it was developed in England and quickly moved to the US, particularly Philadelphia. Henry Deringer was using a similar shaped bolster on his guns in the late 1820s. The form probably spread out from there over the next decade.
Jake and Sam Hawken had adopted a similar style bolster by the mid-1830s and likely used it into the 1840s. They often brazed the bolster onto the breech end of the barrel rather than making a separate patent breech, though.
J&S Hawken also used that shape of lock plate with the radius curve in the transition from the lock bolster to the nose of the lock on many of their rifles that likely date to the 1830s and 1840s.
I would guess that your rifle could date to the late 1830s at the earliest, possibly the 1840s, but the overall style with the pewter nose cap and half stock configuration suggests 1850s.
It's interesting to note that the Hawken brothers also used hollow ribs on their half stock rifles and filled the end with solder like your rifle. That must have been a wide spread practice.
As to the general area that this rifle may have originated, Leman used a similar shaped butt plate and the stock has classic Lancaster lines, but by the 1850s, this pattern of rifle could have been made anywhere along the Ohio River or the Mississippi River from St. Louis to New Orleans. That's almost like saying anywhere west of the Appalachian Mountains.
The cracks in the lug of the breech plug where it joins the tang are interesting. I can't tell if they are indications that the tang was forge welded to the breech plug or if they are from a piece of slag trapped when the plug and tang were forged as one piece. Either way, the quality of the forging and the quality of the filing or finishing of the bolster seem to suggest local manufacture to me.
