Those 13C and 14A "part numbers" don't make any sense in the Lotus bolt specification system. But I just noticed the first line of the #18 item in the parts book is X046B6113Z. So trying to break that down,
X - Hexagon head
04 - # of 1/16" increments in diameter == 1/4"
6 - no idea
B - Bolt
61 - Length in 16ths. inch = 3.8125 = 3-13/16"
13 - no idea
Z - lots of parts end in Z for no given reason
Does that look more or less right? I don't have the car with me right now, but I think a 1/4" bolt fits the hole.