The first, with debossed style lines down the side and a "gas flap", were used on cars from the start of production, with VIN BD000500 up to VIN BD003200 or about 2,700 cars. On this hood, you are able to access the gas filler without opening the hood.
The second, with ONLY the debossed style lines down the side and WITHOUT a "gas flap" were used on cars from VIN BD003200 up through the end of the 1981 model year at VIN BD007199 or about 4,000 cars. On this hood, you must open the hood to access the gas filler.
While no documentation has yet surfaced to explain why the hood changed so often, a few engineering records exist that give the reason as "management decision", with no other details. Anecdotal stories abound about issues with stamping the stainless with regards to cracks around the corners of the gas flap causing a large number of rejected parts, but no "official" documentation has yet come to light.