Hello Mike 2,
yes, here I am again.
I just made a quick phone call to my daughter who is a law-atudent for about four years now and she came up with the following stuff:
1: try to contact a US-lawyers club similar to this one (but for lawyers) and put the question there. She thinks you'll have your answer very quick. There may be even an Alembic-playing lawyer (hey I'm a communication dude so that must be possible).
2: you combine 3 things: you are in good faith, you really tried to contact him, and you have the bass in your posession. She said that (normally) you become the legal owner of the guitar after 10 years. Beware: this period could be longer in the US, but :see 1.
3: the difference between lending the bass to you and actually giving it to you is on this moment not relevant anymore.
4: there could be a problem if the bass once was stolen. But even then she think that on this moment that fact is aged (sorry for my english but thiese legal terms are not really what I'm used to).
5: considering all this you can sell the bass without legal problems (that's what she thinks but still see 1). If you feel not well with that idea and you don't need the money for it you can pass it on to someone who can really use that bass, simply as a gift. That is personal and not for me to advise you on because the greediness will come up for shure. Keep that personal.
So... that's my story!
You're a honest man... It was a pleasure to being of some maybe help.
Paul