Dale & CGW,
I've come to the conclusion that this is the azimuth post for an older (and higher profile) video head. It's similar, but longer, than the post on a 501HDV head. It also could be part of a boom mount. Until the tripod is delivered, I'm going to toss this into the Whatever Pile!
The more important concern is why is this thing still on the tripod?
There was no mention of it in the ad. This is one of those sellers that sell all kinds of stuff - perhaps they couldn't figure out how to get it off the tripod.
OR, it's been on the tripod so long that the mounting screws are corroded and frozen in place.
OR, it's part of a maintenance jig for working on tripods.
OR, they probably took as much of the attachment off the tripod to keep shipping costs down.
I'll post the resolution.
Thanks for your interest,
Dan