Fuel Filler Cap, BF 109 FW 190 Luftwaffe
We have been advised that this is a WWII era Fuel Filler Cap for the Luftwaffe's Messerschmidt Bf109 Fighter, and also used on the Focke-Wulf Fw190 Fighter. Manufactured by Deutche Benzin-Uhrin, stamped "DBU", "Kraftstoff" (fuel), and "Beluftungsventil" (vented).
The corrosion is evidence of its age, but it is in reasonable condition for such an artifact. Measures ~3 inches diameter, ~5 inches across, and ~1.5 inches deep.