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As I've already noted, factory information does seem to confirm that all Mk3 Spitfires were negative ground. But there are some sources around the Net that seem to indicate that the Mk3 dates from January 1967 (I've nothing in front of me to confirm or dispute that date), so ??? Your car is pretty early (FD2081?), but.... ?
I'm guessing that the cables are not original Lucas "Helmet" style terminal ends? A: they usually were clearly marked + or -, and B: they're usually a slightly different size (+ terminal a tiny bit larger, as I recall). If the cables are generic replacements, it's certainly possible that someone reversed a battery sometime in the last 40 years.
Also not conclusive after 40 years, but how are the small wires on the coil connected? Ideally, the small wire from coil to terminal would come from the - side of the coil on a negative ground car. Yes, the car will run either way, but generally it runs better when the coil is wired properly.
I'm guessing that the cables are not original Lucas "Helmet" style terminal ends? A: they usually were clearly marked + or -, and B: they're usually a slightly different size (+ terminal a tiny bit larger, as I recall). If the cables are generic replacements, it's certainly possible that someone reversed a battery sometime in the last 40 years.
Also not conclusive after 40 years, but how are the small wires on the coil connected? Ideally, the small wire from coil to terminal would come from the - side of the coil on a negative ground car. Yes, the car will run either way, but generally it runs better when the coil is wired properly.