When photos of a young Peter Green appeared from his Bluesbreakers period (he had the unenviable task of replacing Eric Clapton on lead guitar) or during his Fleetwood Mac years playing a 1959 Gibson Les Paul, guitarists noted that the pole pieces of the neck humbucker were not positioned correctly – that is, with the pole pieces being closest to the fingerboard. At first it was assumed that at some point, Green simply flipped the humbucker upside down (or backward, depending upon your orientation). That would account for the appearance, but not the unusual tone the guitar produced when both pickups were active. Eventually, Green sold the guitar to another British blues legend, Gary Moore. In the 1980s, guitar builder Jol Dantzig had the opportunity to actually look at the pickup with its cover off and reported that not only was the pickup backward, but the magnet had been reversed, as well. This had the effect of putting the two pickups out-of-phase when both were active, delivering a sound Gibson never intended. Green is on record stating he’d done the modification himself, though he did not specify if he had reversed the magnet (it’s possible this was a lucky “accident” that actually happened at the factory).
Flipping the pickup around physically is a matter of pulling a few screws – not too difficult if you’re careful.
Flipping the magnet is a mod you can do if you’re skilled at doing surgery on a humbucking pickup, although we HIGHLY recommend having such things done by a qualified guitar tech.
Alternatively, you could add a phase switch (either a separate switch, or a potentiometer with a push/pull switch built-in) to achieve a similar sound.