pre-1934 gearbox markings

[JonE]

I read an article by Andrew Bird in an old PWA7C mag, where he came across what turned out to be an original Ulster box. The markings were  X-nnn-X or something like that, with similar, matched, on the gears and I've seen same on early 65 boxes - which I'd wondered whether signified factory replacements.

Can anyone give any more information on what such markings defined - are they sports only, or have people found such markings with standard boxes? On what sort of age range? 

thanks...