Supreme's can have both push fit and screw section fitting - also both button and lever fill - but they all seem to have standard looking open nibs, and the caps are all screw fit.
Diplomas have both button and lever fill - probably also have both push and screw fitting sections (although I can only find screw examples in mine).
Late 1940s examples have the semi-hooded nibs and push fit caps, but earlier pens have the usual larger open nibs and screw fit caps.
I have three lizards, but only in the Supreme. The Diploma was the earliest of the two to be introduced, possibly some time in the late 1920s, with the Supreme introduced around 1940, I think.
Externally, it's the clips and cap rings that show the difference between the two models, but you'd need to see a reasonable quantity to understand the differences - early Diplomas having ball-ended clips, with later examples having metal cap jewels. Later Supremes appear to share the same cap band as the Diploma.
As you can see, there's much similarity and difference, if you know what to look for. Of the two, I think the only design that really stands out are the metal jewelled Diplomas, some of which have a translucent barrel end similar to the Waterman 100 Year Pens.
and if you're not confused by now, you ought to be
Edited to include couple of pix of the later versions of the Diploma - l.f. and b.f., also showing the rather small semi-hooded nib. The black looking pen isn't black - it's a very dark coloured maroon.
Edited by PaulS, 12 May 2017 - 09:06.