I have the BXpanded sprayer for my Kubota 2360, have been very happy wtih it after several uses. I'm not clear if you're referring to the quick-on piece that mounts to the tractor, or asking how you attach the sprayer to the quick-on mount.
The quick-on mount has a fixed pin on the bottom which fits very easily into the pin hitch hole at the bottom of the tractor. The top of the mount is attached to the tractor via the top link mounting hole on the tractor; you use your existing top link mounting pin.
After you have the mount attached to your tractor, the sprayer has a fixed piece on the bottom that slips onto the mount; the BXpanded mount is made only for the BX series, so it's an exact fit on the tractor. One pin hold it together.
The BX mount also fits their "carry-all" which I bought; its an 18" X 32" metal platform which can carry simple tools, bags of fertilizer, etc. Haven't used it much yet.
Have had no problem with the mount; used the sprayer several times, it is sturdy even when filled with 25 gallons of water/herbicide, and easy to remove.
Hope this helps.
Ken