This is a marking guide that fits in a 98mm coupler tube (CT) and allows a 54mm body tube to fit in the center hole.
The guide has marks on the inside and outside edges at 30 degree intervals.
It also has four vertical holes that are meant as drill marking guides for supports rods that go down each side of the bay and supply attachments for shock cords.
Note: one of the sets of holes is positioned exactly midcenter between the inside surface of the 98mm CT and the outside surface of the 54mm BT. This hole position can be used with Giant Leap Slimline motor retainers on the center 54mm BT and still allow for a 1/4-20 forged eyebolt for shock cord attachment.
The other set of holes are positioned closer to the 98mm CT to allow for clearance of an Aeropack motor retainer to be used to retain an electronics bay and still be able to use a forged eye bolt.