Hi, Keith,
I was thinking about finding 73-39 a bit more; could you post an inquiry on Reverb, or on another bass forum, like that which you have posted here: an intent to make a solid offer on #73-39, and float that to see if someone responds to it, while asking for confirmation of the genuine article to avoid a scam. My issue with this suggestion is that I can't think what that discriminating "fingerprint" might be, presently other than the Ox's fading signature on the top, the exact topology of the wood on the top, back, and peghead veneer, the Who Tour decal, and a picture of the stamped serial number on the headstock, something that you can ask to be verified against potential records, maybe. But someone could fake some or all of these with photoshop, and I don't personally know how to scan filters through photoshop to find inconsistencies in the image, if they show up. I also don't know if Alembic can provide certain details relevant to the bass's current condition or location, but I would assume the board should be able to weigh in and help you out with sussing out if a response is a legitimate one.
Although this so-called "plan" is a little riskier than simply finding the owner yourself, using JAWS as an analogy, it's easier on you, potentially, by bringing the "shark" to you rather than searching for the "shark" in open water. The trick is you just have to know how to reel in the right shark. I just dunno quite how to do that 100% safely without you possibly getting jipped. But, maybe it will work even if the owner may be hard to discover, and false positives might spring up. Caveat Emptor, aside, you are in good company (not me) to suss out whether something real shows up, and I think you'll need a bigger practice space.
Cheers!