Our story got kind of injected into the discussion here and in a different thread, so I'll close this part of the story:
I've received a couple of PMs and emails about our number, 21043. Near as we can figure, the POs joined WBCCI in 1970. However, they quit (and we know why: They stopped RVing), and the number was reused by some other folks (with a different rig) for a while. Then they gave up the number for whatever reason, and we got the original number back for this rig.
Oh, and our rig? It was pretty clear when we got it in the 90s that it had been sitting in the field near the POs house for quite a long time. Thank heavens that it had windows closed, but that's about the best we could say.
Again, thank you to all who PM'd or emailed!