Swedish monarchy has a tradition to commemorate jubilees. As i read, the first one was started by Gustav III em 1778. It should be that, the date under the monogram in the escutcheon of your friend's musket refers to the so called
Jubilee of Memory, 1928-
1948 signaled by Gustav V Adolph.
Could it be that this small XVIII century musket was labeled with the 1948 monogram to recall the first Gustav III 1778 jubilee, for some kind of reconstitution ?
... Just a piece of imagination; don't pay much notice
.