Luke and Keith
Thanks but it is the rubber sheet material covering the opening through which the road spring protrudes to the road wheel hub, brake drum (disc) etc, above where the A arm connects to the chassis.
Luke's photo shows that opening immediately above the A arm and the sawhorse and correctly refers to it in his para 2. I do think however, that Aces, Acecas and 2.6s produced both before and after 1959 also had these shields/curtains fitted.
I wish I could post a photo but I am not capable.
Jonathan