some of the earlier dials had index variations, look above at the 'DOMINIO' dial of the late 30's - that is from the FERRITTI collection, the Indexes 9 & 6 are joined and so make a inner, free circle. Something like this could fall out, as it appears it did in the more crusty image posted just above ... this design probably did not last due to the effect. As all of these dials used the plastic filled index concept, I think the flaw was in making the indexes into cutouts ... just not as strong as design concept.
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To further theorise. For some time I have had the inclination that all the BROWN dials are due to the material they are made from. IE. say Bakelite.
Theory). Black dials were brass and painted or stained.
Theory). Brown dials were cast in that colour so common of Bakelite, "industrial brown crap colour". This would also support why and how these dials seems to fade so much more than the the black ones. It was more a result of Bakelite not ageing well, being subjected to light, and age and other factors . . .
Either that or they were made out of coconuts! ...
(I am not being all that silly) components in these pre and postwar times included. Coconut husk, bakelite, and gutta-percha, as well as celluloid, [which I can't see being used at all here!] None of these products are common in our post-Plastic-hydrocarbon lives now, but were commonplace back then.
Gutta-percha might actually be the best choice. Used originally in the 19th century to cover undersea cables, it actually cures in saltwater emerssion.