Great to find an unused set, Bernard.
It would be from some time around 1970, when Airfix added wings and airboxes to all its single-seaters in an attempt to update them, even the previously nice BRM P261 and other cars with the real air intakes outside the airbox, on the side of the engine vee, like the Honda.
Everything also got wide wheels and it was probably then that the motor changed to one with a small front bearing case.
Cars sold singly had 'Plus' added to the name, e.g. Matra Ford Plus and BRM Plus.
Perhaps Airfix also joined the black track brigade then. I haven't seen it before either. Is the section joining system any different?