Hello,
Mr. Gebhard Aders, author of books like "Geschichte der Deutschen Nachtjagd" just gave an answer regarding that unknown plane at the Luftarchiv forum
http://www.luftarchiv.info/viewtopic.php?p=32134#32134
In short, he talked to some Heinkel experts, which declared the photos as genuine without a doubt, one of the experts had held the photos in his own hands.
According to the experts, in the years 1939 to 1940 Heinkel was working on 8 prototypes, it is possible that the photos show one of them.
Regards
Robert