there are 5 designs of tail housings.
The unit with 3 holes from side to side is an F body for the torque arm.
the 2 hole housing is for a bolt on weight used to control vibrations in various applications.
The vette style is a beefy unit , kinda hard to explain the mount on it. bolt holes run top to bottom
There is one with a spot for a tranny mount just like the main case has bolt holes for a mount ( non vette tranny case)
Then a tail housing that has NO additional mounts on it
Some people are like Slinkies... Not really good for anything, but they still bring a smile to your face when you push them down a flight of stairs.