It appears to me the cap illustrated mounts with a backing ring that sandwiches the fiberglass deck between it and the cap portion. There is no picture of what is under the cap and body.

A member who had a Lemans style cap told me that there was an extended filler neck for 63 Corvettes. This extension had a screw flange on both sides and raised the filler about 3 inches. They were priced around $100. At one time I saw them in a parts catalog, but are no longer listed. He said in mounting his, he used one of these to screw his Lemans cap to. A finish seal was made from some kind of foam or rubber material. It looked very good, but I don't have a picture. The price has always kept me from adding one, but think they look great on C-3 cars.