I agree it may have just be a looks thing. Perhaps large amounts of horizontal dropouts were modified like this to use up stock. Perhaps vertical drops were still trickling down from on high. Also fitting the vertical drops is likely a more precise operation so maybe there was s learning curve too
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk