The bosses on the top clamp were designed to match up with a standard BRP submount.
The model I ordered from them on the phone (won't come up on web site) is:
model number: SMO-9018P3 - 90mm spread; 18mm offset.
It looks like this (very nice!!):
Works with Fat bars and a Scotts Submount Stabilizer
Any generic 12mm bolt, two piece, bar clamp set will also work.