damned slow ISP, my post dropped out as I was attempting to post.
Cookey's Mk3 doesn't have the stirrups but the frame as per image.
My old rusty tailgate was sitting of the tray. The reinforcing for the stirrups is flat bar about 1/4" x 1 1/2" about 18" long welded to the top of the tailgate.
Whomever made and fitted the stirrups didn't take much care or use a drilling template. Both sides are different. On the RHS the inner bolt for the end of the stirrup closest the centreline of the truck is within the C section of the vertical rib, but too close to fit a socket on the nut. On the LHS the same bolt is outside the C section but still too close to use a ring spanner. Being pragmatic I'd drill the hole just far enough away from the rib to fit a socket over the nut.
RHS the holes are centred from the end of the top of the tailgate 22", 23", 30 3/4", and 31 3/4"
LHS the holes are centred 570mm, 570mm+1", 570mm+ 9", 570mm+10"
Sorry about the mixture of imperial and metric, but sometimes the metric made a closer full measure.
Hope it helps.