I've just come across this point, so apologies if a reply has already been made elsewhere. It is not correct to say that 71xx should be a first class coach and 7151 a composite coach. The MkIII hauled stock did not follow the same convention of vehicle numbering as the MkIIs. While the EGVs were numbered 76xx and the catering vehicles were 74xx, all the general seating coaches (First, Composite, Standard classes and the Executive Coaches) were numbered in the series 7101 - 7172. There were no vehicles numbered 72xx. For example the Executive Coaches were 7161 and 7162; City Gold (2 + 1 seating) coaches were 7104, 7133 and 7156; First Class (2 + 2 seating) coaches were 7107 and 7157; Composites were 7165 to 7172 and Standard Class vehicles made up the balance. There would have been some variations in these details over the years.