"Does anyone know if anyone makes a screw-in shade for this? I know it would be difficult with the odd thread. Please advise."
Leica made an adapter, coded SNHOO (I think) that would permit fitting 39mm accessories to Summitars. They occasionally appear on fleabay, but sell for much more than a run of the mill step up ring. B+W makes 39mm collapsible hoods, but the ones that I like the most are sold by heavystar (can't remember his exact seller name) on ebay. I think 39mm hoods are also available on Cameraquest.
Another option is a Series 6 to Summitar adapter ring, and by coincidence, I am about to sell one in the classifieds.
gerry