PMT Metal Rocker Box Cap... I think! I have listed this as Potteries Motor Traction as that's the best match I could find. Could be a rad cap or any other screw in cap. Has the usual marks you would expect from its age and getting stuck, so has a few marks around the edge from grips.

Good used condition - has great patina

• Diameter: 11cm
• Depth of front part: 1.5cm
• Total depth: 4cm approx.
• External diameter of back part: 7.5cm
• Internal diameter of back part: 5.5cm
• Depth of back part: 2.3cm
• Tennis ball for scale

I combine postage, so please see my other listings.