28131 331 (which is probably officially -331 or /331) is unlikely to be a Spitfire part. Not impossible at this stage, just unlikely.
If a Supermarine part, which seems very possible given the part number layout, it would be from a type 281, major assembly 31, and be subpart 331.
It is almost certainly a British part as, from my experience, only the Brits used formed tail rivets like in those photos. I have no experience on other European aircraft (except one day on an Me-108) so other European nations may have had the same practice.
As I understand it the way the Supermarine part numbers work, from working on several Spitfires, plus the technical drawing collection I have, and the parts list for the PR XIX, is that the number for the Spitfire Mk 1 parts started with 300 then two digits that identify the assembly (27 for fuselage assembly and 35 for the seat) then a slash that identifies the individual piece. I should note that when the part number is stamped on the part the slash is usually omitted so 30035/55 appears as 30035 55 on the part (or even just 55 occasionally).
So 30000 is the General Arrangement (GA) (or basic layout) for the Mk I and 34900 is the GA for the Mk V.
300
27/735 is the front web plate on the firewall/wing centre section for the Mk 1. This part was used until the type 349 when it was replaced by part number 349
27/7 and the part number of the major assembly (fuselage) then became 349
27. Obviously there are many other changes but this illustrates the way the system worked. There are at least 61 sheets containing 349 prefix parts so that indicates at least 61 parts that differ from all previous types.
Using the seat as another example the basic seat installation as used on the Mk I is 300
35 and the seat mounting bracket that fits on the lower longeron is 300
35/55 or 56 (they are handed) and the bushing is 300
35/57. On the Mk XIX exactly the same parts are used as on the Mk I. The seat however is 39035 and was introduced on the type 390.
Parts that are carried over
unchanged from earlier Supermarine aircraft are also included in this seat - a significant number from the type 224.
The Mk XIX master parts list includes other parts from the types 221, 223, 230, 236, 270, 275 and 285 but nothing I could see for assembly 31. The Spitfire parts themselves jump from 30027 to 30033 so this suggests a component from a different type of aircraft like a flying boat or seaplane or whatever else Supermarine built. I did not go thru the whole parts listing but doubt it includes 31 parts.- as you can see the quality is "poor". If someone knows type numbers for the Seagull or other earlier aircraft the prefix for them would help this person find at least the first type the part was used on. Seagull V which became Walrus was type 236 so type 281 postdates that.
I do not have any drawings for assembly 31 but type 390 assembly 30 is the Malcolm canopy and windscreen installation shown below so the transparency attached hints that they are related.