I really want to know more about this curious gearbox - the selector quadrants are all strange too (1,3,5 and 6 forward???)
All PG1 gearboxes are 5-speed.
The only 6-speed conversion I know about is Quaife - which as Nodge says, is a £6k option, with a more conventional shift pattern (I think???) or available as a sequential.
The Ford IB6 6-speed is said to fit the K-series (the IB5 was used in later Rover 25s and ZRs and there are conversion kits to permit IB5 to IB6 upgrades) - but I am sure that this gearbox would not be compatible with the IRD.
Curiouser and curiouser!