Jim,
I was reading through one of my books on the 356 last night, mainly looking for engine details and noticed that flap of yours. I THINK(?) that you may have a rear carpet piece similar to what would be used on a genuine 356.
On the orignal cars there was a kicked up area at the rear, like a bulged area on top of the tunnel that went upwards and blended into the platform, that your "flap" would have covered over.
I THINK, anyway . . . if you have no luck, I'll scan and send you the picture(s) from the book.
Luck, looks GREAT ! !
OR . . . . it could just be a VIN access flap . . . as Ricardo is about to post . . . below . . .