A slight variation on the split-coil principle is the Z-coil Magnetic Field Design (MFD) pickup, covered under design US Pat. Des. 319,456, filed on May 13, 1988. However, this pickup is a true humbucker. For basses, it was only used for the L-5000 5-string basses, available from late-1988 to early-1992. It never saw the light of day for any 4-string bass! Beyond the integrated housing, one can also see these pickups use only a single adjustable pole piece for each string. This ‘Z-5000’ prototype happens to be one of the few instruments with 2 Z-coil bass pickups.