As the caption for the Gettysburg gun says, it is a cut down Model 1842, a model made as a percussion arm, and so not a conversion.
The Model 1842 was still widely used by both sides of that 1863 battle, in original 3-band long barrel, full stock form and not modified. This example was crudely cut back, and not the product of any "real" gunsmithing.
Just because an item is painted with the name of a place and has been in a collection or museum for many years or is listed in a book doesn't rule out the possibility that it was mislabeled along its history.
And an item that was really collected at a particular place could have been modified afterward, and only later donated or collected (in modified form) some decades later.
There is also the possibility that an item was one of those hauled by the wagonload from various places to Gettysburg for sale to tourists in the years after the War.