Thanks for the reply on Sunday Pete!
It is pre-engaged with, as you say, piggy back solenoid. Garth kindly looked on parts manuals for me and couldn't see any sign of a spacer so a bit mysterious.
I called TRGB who told me that some do and others don't have the spacers. They advised me to check the throw out between the pre-engaged and hi torque unit. If they end up in the same place I shouldn't have a problem, hopefully. I will have to cover the protruding unthreaded part of the fixed bolt with some thick washers.