No ideas, sorry mate. Just chiming in that as far as I'm concerned Sarah Hollett is the better-looking one (admittedly the less buxom), and putting out the links to both threads so others can make their own opinion:
Tracey Harper:
http://vintage-erotica-forum.com/showthread.php?t=8321
Sarah Hollett:
http://vintage-erotica-forum.com/showthread.php?t=8536
Of course, it seems Sarah Hollett never went full frontal, let alone as far as Tracey Harper did.