Rings to convert your humbucker sized guitars to take pickups of different form factors.
Available in Tele neck, Tele bridge, Strat single coil and slanted Strat single coil. Just install the pickup into the ring and screw the ring on like any typical humbucker ring, The easiest way to convert your guitar into something truly unique (For example a Tele bridge pickup in the neck and a Tele neck pickup in the bridge!) without going to the effort of routing out.
Slanted rings make the bass side of the pickup sit further from the bridge (and vice versa for the treble side) to make the bass thicker and the treble sharper - this is as standard on Strat bridge pickups and Tele bridge pickups.