Previously, I posted some pictures of the modular XR-Spectra coil. Two of those coils are held in place by a base printed circuit board (PCB) at the bottom. The two coils are connected, series opposing, for a humbucking configuration. The outputs are sent to a 4-pin, 2mm header connector at the bottom, with pins for coil-, coil+, middle-coil, and ground. Three Neodymium block magnets are placed at the bottom, with the magnetic field coupling through the two rails in a U configuration where the top of the rails having opposing north and south polarities with an average flux density of around 160 gauss (16mT), measured 3mm from the top of the rails (approximate distance from the strings to the top of the rails).
For comparison, the Fender Stratocaster single coil was measured to have a flux density of around 217 gauss (22.5mT), measured 3mm from the top of a pole. The comparatively lower flux density was deliberately chosen to minimize the stratitis effect (warbling sound with dissonant frequencies heard as you play further up the frets, which happens when the pickup is adjusted very close to the strings).
Here’s another view of the double coil assembly: