I thought the number 60 was from the thickness or diameter of the imput shaft but I was mis-informed or got confused. A thicker imput shaft like an axle could handle more torque with all else been equal.
To save on tooling and resourses Toyota used the same gearboxes with different bell housings for it T and W series box.
The 60 series L based 5 dogleg box I have has front housing inc. bell with the same casting patterns, shape or design with the oil drain and fill plugs mimmicking the 180b SSS dogleg. There must be some chum with this knowledge out there who is not on this site or uses the web.