Looks like chassis number is 73082184P.
Quite how that makes it a 1974 car, I can only guess it sat in a dealers for a few months before being registered.
Lotus-Ford won the GP World Championship for makes, but that was not until October, so the badge on the side with 1972 being the last, would have been correct when it rolled off the production line.
IMHO, a 1974 car would be built in 1974, and have the 1973 WCCC badge on the side, from about chassis number 74012340 onwards.
I agree that £15,000 is a very "full" estimate of the price it should achieve at auction.