Yes I think you have confirmed my doubts, just looking at various pictures it does appear that the single diagonal bar version has vertical uprights as opposed to inwardly sloping ones of the double diagonal. Just looking at the Thundersports half hoods on eBay it does actually state that they are not compatible. So I think that is the answer. I have also sent an enquiry to Caterham for clarification as they advertise only one type of half hood for an S3 FIA roll bar. I will post their reply for the purposes of completeness. Thanks for your input.