I've fished out the British equivalent (
it's a Monster) and noticed some string through the insulator mounting holes. Turning it over, there's a cork gasket there, to seal the joint between the aerial base (Insulator W/T H) and the Tufnol plate. There's then a cardboard (I assume grease or varnish impregnated) square laminated gasket with only 4 holes (at the corners) to seal the joint between the Tufnol and the vehicle roof.
You'll need a circular cork washer between the top half of the C2 insulator and your square plate to prevent water ingress, as well as the square gasket under the plate.
The British plate only had four bolt holes holding it on the vehicle because the base insulator flange is 8.5 inches in diameter, with 6 holes to bolt that down.
I need to photograph it (and all its related parts), if only to frighten people, but I'm still missing some of the 'F' rod adapters for use on the move.
The drawings in Wireless for the Warrior Volume 2 really don't do it justice.
Chris.