Otis A base Test board slot

I am trying to help out a friend who has a older Otis A,B base controller..he needs ro find out what slot the test board safely goes in to the car operation controller.I had the b base years ago but my board slots were xa,xb,etc..and we used the yk slot.his vintage is different it has the slots A thru Z..need to know whicj slot this goes into for troubleshooting..any help appreciated..

Slots are slot "T" and slot "X..

The board with DO goes in T

........ok wanted to make sure that he installs it in the right slot so he doesn,t short anything out....x it is..thanks