Something got me wondering, what standards detail how a "NEMA head" should be constructed, and also if the standards were actually written by the National Electrical Manufacturers Association or if it is just a misnomer.
I think that when people say "NEMA head" are referring to interchangeable latch-on optical assemblies that aren't just found on bucket lights, but also on gumballs, radial waves, teardrops, admiral's hats, and the pendant style Westinghouse OV-20.

Can anybody give me links or to any standards that define such a head?

This could be what "nema" refers to on this website: https://www.nema.org/membership/products/view/luminaire

IIRC some of the luminaire and ballast manufacturers referred to it as "EEI-NEMA Dimensional Standards". I suppose it referred to features on the head such as the outer dimensions, particularly the dimensions for the latch on optical assembly. In addition there were also prescribed dimensions for interior for the ballast/socket mounting bracket but this standard seems to have been relaxed with some NEMA heads using a yard light style ballast mount. Originally the socket/ballast mounted to two threaded holes near the top of the NEMA head and ballast kits were made to make use of those mounts. This allowed incandescent NEMA heads to be retrofitted to MV, MV to HPS ets.

As you mentioned, there were plenty of NEMA compatible optical assemblies, from the usual 12" bucket to gumballs, tear drops, radial waves, 16" PowrBracket buckets, admiral's hats, and even FCO optics like the Hubbell Skycap. Even some early clamshells such as the OV-20 and the Form 109 had a NEMA compatible or a adapter ring.

IIRC some of the luminaire and ballast manufacturers referred to it as "EEI-NEMA Dimensional Standards". I suppose it referred to features on the head such as the outer dimensions, particularly the dimensions for the latch on optical assembly. In addition there were also prescribed dimensions for interior for the ballast/socket mounting bracket but this standard seems to have been relaxed with some NEMA heads using a yard light style ballast mount. Originally the socket/ballast mounted to two threaded holes near the top of the NEMA head and ballast kits were made to make use of those mounts. This allowed incandescent NEMA heads to be retrofitted to MV, MV to HPS ets.

Do you know where I could find said standards?