Assuming you're correct in saying that the D and E are identical:
Enter it as a D model. If anyone kicks up a stink, say "yep, it's a 1977 model with a '78 frame" over and over again.
Eventually they'll work out that they're objecting to some numbers stamped into the frame and nothing more.
If they keep pushing the point, then point out that the frame is a carry-over part, and produce the copies you've got of parts lists and or brochures...
The problem should go away quite quickly then.
If you're really paranoid, the obvious answer is to tamper with your frame number...