two possibilities - Japanned enamel, or early type I anodizing.
Shakespeare was definitely anodizing in the 30s, the example being 1900 Steelhead & Trout fly reel, Model HF.
Thin type I anodizing is literally only a few atoms thick, and tends to look bronze or even purple in thin spots.
Here's a type I anodized Martin showing the bronze appearance.
Shakespeare also anodized in red for South Bend (SB 1185, also 1935)
Japanned enamel is a slurry of black iron oxide in a carrier, furnace-baked, and while it makes a spinel bond with the aluminum base metal, is easily scratched.
