[Kube]

Clem,
The saying goes "it's all in the details". Those details can often be found through diligent research at the Benson Ford Research Center, a literal treasure trove of documents.

During my research for the '40 Ford book, I'd not found even the slightest hint that the generators were anything but gloss black.
As the '40 Mercury utilized the identical generator as the '40 Ford, I'd say they too were always painted gloss black.
Unless newly discovered documentation comes forth, I'd require a generator to be gloss black during concourse judging.

As far as "...generators are frequently changed"...judging upon a (serious) concourse is aimed at determining how the vehicle left the assembly line.