The F/M/J spindle is 3/8 of n inch taller overall than the 71-72 B/70-74 E/ 73-75 A body spindle.
This was changed to allow for the additional height of the 73-79 B body only ISO front K frame bushings.
This is not the thickness of the BJ mount.
The discussion concerns what that additional 3/8 does to the geometry.
Some say it hurts (potential bump steer), some say it helps (caster angle).
F/M/J (and 73-79 B which are the same part) are MUCH easier to find than the 71-72 B/ 7-74 E/ 73-75 A part.
