No longer being made, but here’s the info from the Pickup Wizard website:

To reduce costs on this budget model bass, Fender often used a Musicmaster or Stratocaster 6 pole guitar pickup hidden in a solid top black plastic cover. Using a 6 pole guitar pickups works but it is less than ideal. What we have done is to custom develop a four (4) pole version which matches the 2-3/16” Musicmaster Bass string spacing. By lining up the poles directly under the associated strings you get more punch and articulation. Additionally, we also use slightly longer magnets which allows us to boost the output from the typical 5.7K ohms to approx. 7K++ ohms again enhancing the tone. The pickup is about 1/8” taller than a standard pickup but will fit into vintage or re-issue Musicmaster Basses with no modifications to either the body routing or the pickguard.