The heart of the Mark VI is a semi-space frame chassis. Rather than a complete car, it was available to the general public as kit, wherein the customer could install any preferred engine and gearbox, making it eligible for a wider number of formulae.
The Mark VI's core component is a semi-chassis frame, not a finished vehicle.  It was offered to the public as a kit, enabling them to choose their desired engine and transmission, expanding its applicability to various racing categories.