I had one which IIRC was October '71 registered and used by the factory for the first 2 years of it's life, although it wasn't a press car.
BOP is no. 732 (December '71) & my previous 'S' was no. 668
I got rid of it so long ago that I don't think it's still listed on there. I just tried to check again and the DVLA site is down.
It's the prototype for what would become the 3500S, based on a 3500S Automatic.
Prior to that, several of the development P6B's had manual gearboxes.