Now these Negretti & Zambra badges really are quite something, lozenge shaped and resplendent in a a mazarine blue vitreous enamel ground over a chromed brass backplate of size 63 mm x 25 mm thee are indeed quite special. . As you can see from the photos, the badge has a very slight curved radius to allow it to fit not just a flat surface but also a slightly dished one too.
I have one of these on my Fortin barometer casing and very nice it looks too. A rare item, rarely seen and new old stock - limited number available.