FF
Usually engravers decorate the part or parts of a item i.e the sideplate usually made by others,the one shown could be made and carved by an engraver. IMHO
In the time period these plates would have been manufactured they were hand crafted by the makers themselves-a side plate is basically a washer-so gun makers began to shape them and sometimes add sculpting (what engravers were called in those days) Many of the "sculptures" of that time period were and are quit well known for other works-Michelangelo-Da Vinci to name but two sculptures who also worked in other media. Fred
The gun in the photo is probably an English style fowler, mid 18th century (possibly a blunderbus). Various trades supplied components to the gunmakers. The sideplate was probably made by a silversmith. If you could remove the plate and look at the back, you would see two or three stamps that indicate the makers touchmark and a code for the year made. The European gunmaking trade was quite different from American gunmaking, where the gunsmith, especially the frontier gunsmith, tended to make as many components as possible. They would cut sideplates out of brass or iron and may or may not bother to engrave them. Gunmakers in major metro areas may buy more components because they they have access to them.