The bridge pickup is "FAT" wound on a deeper bobbin. This enables the use of a lower gauge wire giving a fuller more rounded sound.

 

Because of the thicker wire the K reading is lower than your average XII pickup.

This does not mean lower output!

With the higher inductance, this 6K pickup is the perfect match.

These are advertised as new other.. they were briefly in a guitar, being my 1st attempt at a fat bridge I was curious about the sound.

I was not dissapointed.

These are humbucking in the middle position. 

https://www.axesrus.co.uk/Pickup-Covers-Suitable-For-Fender-Mustang-Bass-p/mucover.htm