To add more fuel to your fire...
A Pre-L engine cover (the center piece)is "all screen", and curves down toward the glass with the same curve as teh side covers.
An "L" center cover has a solid piece in the center, (with the wire mess on either side), and the front section towards the glass is in the shape of the air cleaner, somewhat.
I can send you an image of an "L" cover if you like.
Then during the L production, (which started in August, 1972, with the bumper change, and other items...) the side engine cover pieces also changed. The gas filler was reloacted into the gill, officially with VIN 5000,(but my 4580 has it) and shortly thereafter, the engine cover on that side was changed (perhaps twice) to cover the gas fill hose.
To ramble on....
Then quite a while later, the right side cover was modified. (Early or Mid production 1974s ?) A hole was cut into the cover to give better access to the water fill tank.
If this is not enough, early 1971 cars were different from later 71 and 72 cars, where the engine screen meets the window.
How the cover fits into the groove was changed at some point.
I think some modification was made to this groove at the "L" change over also, so going from the early type to the L type may require some slight mods.
Need more, or was that too much already ?
Chuck