That small adapter plate needs to bolt directly to your mitsu case.
This may clear it up a lil. The red line in the pic points to the countersunk holes where the second case(for you, the mitsu) would need to bolt up. That 6-bolt pattern is different than the mitsu pattern. You'd need a plate with the mitsu pattern drilled, and the innercircle of smaller holes to bolt to the other adapter plate(which then bolts too the 231).

The red circle is where the coupler shaft would need to go into the input of the Mitsu case. That part of the shaft is 23spline, so it might fit your case.



hope that makes sense
<img src="/forums/images/graemlins/cheers.gif" alt="" />