TODO(kurt): Find out whether the RIGHT plug-in slot is special in 11400 and DSA600 scopes. It seems likely that it is, given that the 11T5H manual says (section 1-1):
The 11T5H is used with Tektronix Digitizing Mainframes.
and lists only 11400 and DS600 mainframes in the mainframe compatibility table (table 4-4), and the same document (section 1-7) says
Note: If the 11T5H is to be used to trigger on signals originating from an 11A34V Video Amplifier, the 11T5H must be installed in the rightmost plug-in compartment of your mainframe.
So it seems likely that in 11400 and DSA600 scopes, the trigger signals are routed to the RIGHT plug-in slot, same as 11300 mainframes. However, unlike 11300 mainframes, 11400 and DSA600 mainframes allow the RIGHT plug-in to function as a vertical plug-in, i.e., its main signal output (pins 11A/11B) can be used as the vertical axis.