The C series boxes have the bolt on plate around where the shifter goes in, this is what I'm after (like the box on the left in the picture below), the B series boxes have the shifter held in with a clevis pin type arrangement.

This is a picture of the short C series box on the left compared to a stubby B series box (L20ET/R30/C31).

This shows the difference between the two lengths of C series box

This is a stubby B series box next to a long (RB) C series box