This piece hold the rear bearing in place inside the trailing arms. They are identical for either side. They appear to be originally yellow zinc plated, according to the set I recently tore down. The plate is held in place by four yellow zinc plated 8mm x 1.25 x 15mm bolts with yellow zinc split lock washers.

Bolts - 900.075.078.02
Washers - 900.027.015.02

The PET illustration shows this part as being round, but as the images show the actual shape. I haven't seen a round one, including the original 914-6 ones I've worked on.