Simon,
I think it’s been varnished at some point with the reddish hue to the front, a cotton bud dipped in some turps or paint remover (Dab n wipe, dab and wipe) on a small area may uncover it’s original finish, painstaking but sometimes worth it.
The reverse looks like it’s been filled with a pine pitch or similar, a heat gun should help melt it for removal without any damage to the metal, the central device may come out easily then for correction and I’d hope you found a nice badge underneath it all
Paul
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