I noticed a lens in an ebay shop which shouldn't exist: A 285mm f:6.3 Zeiss Protar. The shutter looked too small for it to be a 285mm f:6.3, but too heavily repaired to identify (!). Pictures of the front showed "Protarlinse F=285mm" engraved on the bezel, pictures of the back showed no engraving at all.
I got curious, and bought it.
The engraving on the rear cell is around the edge, and says "Anastigmat 1/12,5 F=285mm", no serial number but the same patent number. Which makes it two cells of different age but same type.
The only possible candidate is the "Serie VII" Doppel-Protar, two 285mm f:12.5 cells making a 160mm f:6.3 combined lens - and a far better lens than I thought I was buying (and the seller thought he was selling)!
Were you able to find a listing for 285mm? I couldn't find it in my catalogs, and my 6 element set has 29cm. It may fall between the 1891 and 1936 lists I looked at.