The Indianapolis sister cars were the Mk2, with subsequent Indy designs also taking model numbers 3 and 4. The Mk5 was a Formula A car adapted from the Mk 4 chassis, while Mk6 was the designation given to the still-born Formula One successor to the Mk1.
The Indianapolis siblings included the Mk2, as well as subsequent Indy designs designated with model numbers 3 and 4. The Mk5 was a Formula A car based on the Mk 4 chassis, while Mk6 referred to the unsuccessful Formula One successor to the Mk1.