Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules
Hmm. Early Spits - just a stab in the dark, but maybe a test instrument of some kind, such as a 'g' meter or early form of stress/fatigue meter ?
now that I look at it again, I see the empty ring.
Found this here but it says the photo is of a Mk.II...Spitfire Mk. II Details — Historic Photographs | 1940 | Reference | Spitfire Mk. II