The broken off top of the piece could possibly have added a couple of notches, so it could represent a 5 or 6 string bridge. The apparent size of the notches suggests a fairly heavy string, and for a good sound, even to prehistoric standards that might be expected**, a fairly long string would have been necesary to be compatible with the implied string diameters.

Conclusion: It's a bridge off a 5-string stand up bass.

Next question: Where's the %@$@#! banjo for the lead player?

** Development of a "sophisticated" instrument of the kind suggested by the piece would have been unlikely unless a "pleasant sound" was produced, so unless the diggers can come up with a bow the strings would have to have been long enough to "sound" (at relatively low tension) with some "strength" when plucked (or whacked hard?). The multiple notches - if they're in fact for strings - suggest a multi-tonal "music" although speculating about harmonic relationships between multiple strings in that time is something of a stretch until additional evidence can be produced.

Maybe there's a juke box back in the back of the cave somewhere, so that someday we'll hear their recordings?

John