When we install many instruments (for example telescope with guide scope in parallel) on the mount, it can happen that they are too close together, and that some element touches the others. That's why we created the PLUS 10mm spacers that can be fixed between two mechanical elements (such as a plate and the support rings), keeping the functionality of the PLUS system. In fact they have four holes, with 20mm separation to fix PLUS Vixen dovetail bars and guide rings and 55mm for PLUS Losmandy dovetail bars.
The PLUS spacers have holes to fix the other elements always with M6 screws but of greater length (25mm) which are provided in the package (4 screws). In the image above, the PLUS spacers have been installed between a PLUS Losmandy dovetail bar and PLUS support rings.
I bought these for mounting the 95mm rings for my FSQ-85 to a 350mm Losmandy type dovetail. These spacers are needed as the dew-shield and the focuser would be in the way for longer dovetails. As the FSQ with imaging equipment is very back-heavy longer dovetails are more convenient for balancing.
These spacers come with longer screws, so mounting the tube rings to the dovetail was easy and the PLUS system makes it very solid.
If you want to mount a longer dovetail on top of the FSQ then higher spacers would be needed, so it can pass over the focuser-lock.
I think the 30mm would be best to still be able to use the focuser-lock mechanism.