Vintage caseback variations

I’ve been examining the caseback of my mid-1940’s 2421 (with cal. 28.10.RA.SC.PC ). It looks different from the other 2421 casebacks I’ve seen on OF. I initially thought mine might be not genuine. In fact, I see variations in pretty much all the 2421 casebacks (inside markings, cripness of markings, fonts, shape of caseback, perlage, etc.). Amongst the 5 examples below, are they all genuine? Were the casebacks made by several providers to Omega?

Specimen 1



Specimen 2



Specimen 3




Specimen 4



Specimen 5