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.
In the context of Indianapolis racing, the sibling cars were designated as the Mk2, with subsequent Indy designs also assigned model numbers 3 and 4. The Mk5 was a Formula A car developed from the Mk 4 chassis, while Mk6 referred to the unsuccessful Formula One successor to the Mk1.