The "M" family of pickups was developed primarily as replacements for basses with existing Bartolini and EMG "DC" pickups of the same size. However, these pickup models were also popular with luthiers building new instruments with these standardized pickup sizes.

M-family pickups can be used alone, or in pairs. They were made with 3 different coil types, for different applications.

Known Models
Coil design and aperture3.5 inch4.0 inch4.5 inch
Humbucker (parallel, 2 conductors)M3.5HBM4.0HBM4.5HB
Humbucker (split-coil, 4 conductors)M3.5HBSCM4.0HBSCM4.5HBSC
Single coil (narrow aperture)M3.5NM4.0NM4.5N
Single coil (wide aperture)M3.5WM4.0WM4.5W

M-family pickups share a common set of housing sizes, and can be used interchangeably in basses which are routed for that same pickup size. All M-family pickups are 1.50 inches in width, and have a 1/8 inch corner radius. Usually these sizes were optimized for common 4, 5 and 6 string basses, and differed in varying increments of length.

It is difficult to tell which type of coils are inside a M-family pickup, unless you remove it from the instrument and look at the engraved model name on the rear of the pickup housing. Unlike the SoapBar family of pickups, the mounting holes do not usually vary based on the pickup's coil type(s). If the inscription is not visible or you do not wish to remove your strings and pickups, you may use some form of magnetic viewing paper, which will show the shape and width of the coil(s) inside a M-family pickup.

By contrast, it is relatively easy to determine the rest of the pickup's model number: you only need a ruler. Measure the length of the pickup in inches, and keep only the 2 most significant digits. For example, a M-style pickup which measures approximately 4.0 inches in length must be one of the following models: M4.0N, M4.0W, M4.0HB, or M4.0HBSC.