Many of the British cars through the late sixties were positive ground cars. The positive, not the negative, connection of the battery was grounded to the chassis. I have no idea why the Brits did it this way (Ford and a few other US manufacturers had positive ground systems in the 1940s and early 1950s), but I'm sure there was some sort of jolly good reason.
Despite their reputation for electrical failures, older British cars have fairly simple wiring. If you have any automotive electrical knowledge, getting this car back on the road could be a weekend's job at worst.click here to see the Craigslist ad.
PS - You can convert a positive ground car to negative ground. But, why bother? Keep it original. Just look over the shoulder of anyone trying to jump start it.