PART NO.  SPL.PAGE  DESCRIPTION                                                           
ATB7078   15.H.4    Plate -brake -R/H -(rear brakes, 1500)
ATB7078   16.H.4    Plate -brake -R/H -(rear brakes, 1600 & 1600-MK-II)
ATB7079   15.H.4    Plate -brake -L/H -(rear brakes, 1500)
ATB7079   16.H.4    Plate -brake -R/H -(rear brakes, 1600 & 1600-MK-II)

Rear brake backing plates for MGA. Difference between left and right is orientation (clocking) of the four mounting holes at the center hub.

There is a brakes tech page BT-102P on corrosion problems with these parts.

Also a Faulty Parts page FT-044 on how to deal with a rear slave cylinder that does not mate properly with this backing plate.