what's at the top of this stand?
 

Matthews | Digital Combo Stand - 11' | 369675 | B&H Photo Video

Is this the "integral rack mount adapter"? If so, it's not the kind of rack mount I'm familiar with.

No no. It's a baby pin adapter.

Thanks. How does the upper tube end? Is it threaded?

There's nothing threaded on a stand. The top of a combo stand is a female receptacle to accept a junior fixture. On a Junior, the fixture has a stud and it fits into the stand. On Baby fixtures, it's the opposite. The stand has a pin, and the fixture has the female part that fastens down.

On a combo stand it needs both. On my combo stand, it has a little release that pops up a spring loaded baby pin if I need to attach a baby fixture to it. But putting a baby fixture on a junior stand is like using a large truck to carry a suitcase. It's just WAY overkill.

Interestingly enough I've seen some stands that have a short 1/4 - 20 male stud on top of a regular baby stud.

True, and I have 2 of them. Forgot about that. I use them to hold foamcore as I have two gator grips that have theads for this. Seems to be a Manfrotto thing.

I find the 1/4" 20 threaded post incredibly useful. I even tap my other 5/8" light stands with a 1/4" 20 hole so I can put a stud in if I need to.

OK - I'll admit it. I once used the 1/4-20 stud on a light stand to put my still camera up about 16 feet with a wide angle lens on it.