Spot on. The P6B was developed right at the end of Rover ownership of the company, just before the takeover by Leyland Motors (the lorry co who already owned Triumph) and subsequent merger with BMH (formed of BMC and Jaguar). Prototype V8's were running in early Talago shells - the car owned now by Ian Trapp is just such a car - and preproduction prototypes were on early D plates. By this time the tooling for the front valance was complete, and whilst the new valance for the 4 cyl car managed to creep under the wire of the subsequent investment freeze, rejigging the V8 one to match did not. Thus the end sections of the V8 undertray match those of the sharks tooth 4 cyl one. The 4 cyl undertray was redone as a result of rallying experience in the '65 season care of Roger Clarke and freinds. I would confirm that there is a noticeable difference in front end stability between sharks tooth and later type valances, thanks to experience with our '65 car which wore both at various times thanks to an incident with the back of a sand lorry on the A30...... I've not yet had the chance to do a personal comparison of the V8 with and without spoilers, but hard drivers whose opinion I trust say that there is a small improvement in lift and a much bigger one in susceptibility to cross winds.
Chris