After a little more thought, here's a hypothesis (based on nothing) about how this decal got on this one-string violin:

None of the autoharp manufacturers, Dolge, Phonoharp, Schmidt or anyone else, made their own decals. They bought them from some supplier. They may have been designed by autoharp makers or maybe not. In any event, whoever made the one-string violin in question simply bought one and stuck it on.

Why not?

Bev and Jerry