Found a leitz "close up adapter" for sale... says that it's specifically for the 50 summicron. What exactly does it do and how close-up is "close" ?

Is this a kind of long frame with 3 apertures (left and right ones with lenses in them, middle one empty)? Are there 2 short levers projecting from the front? If so, this is probably the attachment for a close-focusing (or "dual range") Summicron, to which it would attach by a dovetail fitting to enable focusing between 1 meter and 48cm. It won't work with a normal Summicron!

It could be for the Collapsible Summicron. The collapsed lens fits into the adapter, which has its own Helical.

It's got a few bumps and/or adjustments around the perimeter...
Hopefully this photo attaches correctly. If not... it has a square frame at the top with a small ring adjacent. there are two knobs and some sort of screw adjustment / stop on the perimeter.

From your description, I agree with lens_hacker - this adapter interposes between the lens and camera body and allows rangefinder focusing with parallax compensation between approx. 1 meter and 50 cm. The adapter came in various models, SOOKY (or Part No. 16502), which accepts a collapsible Summicron in its collapsed state and is for screw-mount cameras, SOOKY-M or SOMKY (16507), same as above but for bayonet-mount cameras, and SOMKY-M or SOMKY-UOORF (16509), like SOMKY which accepts the head of a non-collapsible Summicron. An entertaining accessory if it is not too expensive and you already have the appropriate lens and camera!