This is the most popular size of backing plate, and the 'cushion' design is favoured by many bodyshop pros. The 25mm ultra-soft foam sandwich helps keep the pad as flat as possible during use, maximising the contact area of the pad and increasing the consistency of the finish.
The Midi Mount works a treat with all our 150mm plain, fin and waffle foam pads. Or 150mm/6 inch pads (and larger) from anyone else. They don't have to be ours. Just ensure that they have a 'hook and loop' velcro backing.
The thread is M14 rotary, so don't try and get one for your DA. It's not going to work. And it can spin up to five times faster than a normal rotary machine polisher at maximum attack, before it melts into atomic sludge. That's a lot of rpm.