... is that Lange used this unique movement calibre -- the Sax-0-Mat movement with an added power reserve feature -- just for this dealer limited-edition. Lange also used a unique movement for the Wempe 1815 "Sidestep." The 1815 "Kalenderwoche" for dealer Huber was a third example of a dealer-only movement calibre.
I think the Wempe Grosse Langematik Gangreserve is also the only automatic that Lange ever made with a power reserve complication. It allows the wearer to see just how efficient the Sax-0-Mat winding mechanism is: When you wear this watch, the 48-hour power reserve is almost always completely full, with relatively little movement.