I wouldn't be too certain that it's the original fork, given the "SB" stamping. That is likely an Ilkeston SBDU (Special Bicycle Development Unit) fork from a Raleigh Team Professional.

Given the Team-inspired paint job, I'm not surprised that the fork found its way to that frame either; probably a whim of the previous owner to have something resembling a Nervex-lugged Professional. That they repainted it with the 1984 Raleigh USA Team 555/753 paint scheme is equally ludicrous. A mutt in the worst sense of the word, restomodded. Yech.

-Kurt