Astro - Kit Plate Comparison Photo by 65Gladiator | Photobucket
This picture shows about how my backing plate looked after the modification (photo on the left). The booster itself attaches to the backing plate with a large threaded sleeve which slips through the hole. A large nut captures the booster and tightens it against the backing plate.
You can also see the notch which indexes (clocks) the booster unit against the backing plate. there is a machined groove on the booster unit to make with the notch. In my booster, it had two grooves machined and two notches.