Unusual thing! Type 3B Servo Altimeter by Smiths Industries of Cheltenham. Rated from -2265ft to +40,000ft, putting it out of the range of propeller driven aircraft. Had also been stamped as having "Mod" codes done, possibly indicating that it's a flown example, as does the included instrument panel mounting bracket.
I know very little else! If you need any more info, please ask.
