If I am seeing the picture correctly, those pieces of ply are doubled...two make the thickness? Could be old eyes. If that were the case, they might be staggered, like a half lap splice?
Regardless, totally agree with titebond and a doubler. Your main indicator would be the bulkheads. If there is a doubler there, there ought to be a recess in the bulkhead to fit it. Might help ID the thickness of the doubler as well. I suspect it would be something harder than balsa as well.