Your rack is the original type supplied by MMC.
The brackets with the pins are bolted to the front of the rack. The other two brackets with length of tube welded to them are bolted through the roll hoops, you will need to drill the roll hoops to fit. The pins that you have bolted to the front of the rack then go into the tubes bolted to the roll hoops.
As Planenut says there are alternative ways to fit without drilling the roll hoops, using roll bar clamps or homemade clamps.
