Can I suggest you post this question on 10-Tenths in the Historic Section - sorry if this offends the rules but there is a specific thread on there about all the 6R4 cars and their history and that includes some contributors who either have researched the cars history extensively or have worked on them in the past, some even in the Austin Rover Competition Dept at the time. They may be able to help.

Bizarre though it might seem this place http://www.centralmotorcycles.com/ may be of interest. The owner's job in group B era was one of the international spec engine builders in the works department.

He's quite a quiet chap it would seem though.