Based on my rather small corner in space (the NE) and time (the mid-1930s), I am tempted to say "varied from painter to painter".  I dunno, maybe the  B&M painters were really enjoying their Guinness (or whatever was available back then), but it seems much easier to find multiple cars that were lettered differently than even two that were exactly the same.

Of course, this makes my life easier when working up (or buying) decals:  "close enough  is good enough" is a helpful motto when you have a handful (at most) of prototype photos to represent a car series numbering in the hundreds or (sometimes) the low thousands.  As always, YMMV.

Dave Parker

Riverside, CA