OK I couldn't resist this one so I had to get off my butt and go downstairs and check it out. I knew the the FI and carb plenums are different but wasn't sure why although I had my suspicions which proved true. I have a '79 carb and '80 FI car. Although the top one seems to have a rounder bump that is likely just the lighting. The bumps are the same shape. The FI plenum bump is shifted to the right/extended (when on the car) to make clearance to the bonnet latch which is shifted to the right. The body has two cut outs to aid in release of the latch when the release cable breaks, one regular (carb) position and one to the right to match the shift in latch position. The '79 carb car has just one cut out as they were all carbs then.
The top one is indeed an FI plenum. I suspect the reason you have three in the UK (baring someone bringing in a batch of US cars) is that they used what ever was handy on the production line. Wouldn't be the first time.
No, all the edges/flanges are exactly the same on both, its just that bulge which is a different shape. I kind of suspect its just as FI Spyder suggests that they just used whatever came to hand on the assembly line.
The one in my 81 FI was a bit ill-fitting. Slight "modification" with a BFH was the business of the day.
I thought about moving the latch and the catch, as it looked like it could easily be done with the existing holes and captive nuts on the fire wall and the hood/bonnet. But, the hammer was easier, and much more fun.