Yes the existing is not a real NEMA fixture, it is just a plain bucket light....the NEMA head is a specified fixture (excluding the bucket) which is specifically sized at 8 inches diameter lip with 2 clips (and sometimes screws) the purpose of an NEMA head is so the refractor and reflector is to make it easily interchangeable, you would be able to put an old gumball glass on that AE NEMA head (by simply removing the bucket and put a gumball) or even a tear drop or a radial wave....
NEMA doesn't really mean a "Bucket light" the NEMA head is really the top part of this whole fixture.
Let's not forget that the vertical version of the OV20, the Form 109 and early style Ovalite with adapters can fit this head as well. The GE Form 400 used screw rather than clip mounting.
NEMA doesn't really mean a "Bucket light" the NEMA head is really the top part of this whole fixture.