EK, Furnishings could have been plain, off another gun, or imported plain. Engraved here.
Rich, I don't see the box as a different hand from the other parts. The borders are wobbly on all parts, the buttpiece is engraved to 'look like' high art engraving, but it's quite crude, esp in the shading. Same for the sideplate. Subject matter is right-on for the period, but the draftsmanship is wanting, and the shading is not regular or well practiced. It goes from shallow to deep with little rhyme or reason.
The box is again, certainly european influenced in its decoration. It's also the crown jewel of the gun, a canvas where personal subject matter can be employed. Again, it's crude, same unpracticed shading, crude draftsmanship, same wonky borders as BP and entry thimble. Since this piece is such a focal point, the artist lavished more time at it. More complex artwork, more imaginative imagery, but the hand didn't change, IMO. Maybe the box was in memory of his life on the Continent?
I don't remember seeing any castles in 1740 America, unless they were Iroquois or Seneca.