Iver wasn't the only Johnson to make guns....
My listing of American Firearms Makers has 9 Johnsons listed, and these are only a few of the possibilities. Going by the fact it is a flinter, it probably was made prior to 1850 or so. However I believe that flintlocks were still being made much later. (They weren't dependent on having caps to function. This could be important in the wilderness.)
Using 1850 as a cut-off date there still 5 possibilities:
Henry Johnson circ. 1840 Buffalo NY
Robert Johnson 1822-1854, Middletown CN.
Seth Johnson 1773-1777, Old Rutland MA.
William Johnson circ. 1790 Worcester, MA.
Samuel Johnson circ.1850, Pittsburg, PA.
Of course there were a large number of "part time" 'smiths that made guns as a sideline to their other profession. (Usually farming. They made firearms in the offseason.)
The named lockplates are probably because that there were makers that specialized in locks which were purchased by gunmakers for use on their firearms. Why go to the effort of making a lock, when you could buy a ready-made?
My listing shows a Daniel and Jacob Christ as Kentucky Rirle makers in Lancaster PA in the 1770's. As professions tended to run in families, Brandon/Landon Christ may have been a descendent or other relative.
As to value, I will leave that to someone else after you post some pictures.
(I know nothing about shotguns.