More Help Please - Rear brake caliper mounts this time

Hello!
I think I've worked this one out but if my life and all it's ups and downs has taught me anything it's that assumptions turn into the mother of all **** ups. Your assistance is most welcome at this point.

I'm starting to offer up some of the parts I've been accumulating for my 1978 frame. Today it was the turn of the rear brake calliper mounting plate.Here is what it looks like on the bike.  

Here is what it looks like on the bike.

This is what the 1978 parts catalogue looks like.

I have got the wrong part ain't I! What I need is a 83-7018 Mounting plate, Master Cylinder don't I.

Now I have to assume (see above caveat) that the lug on the frame is the location of the bolt which holds the back of the brake plate in place. The other two holes in the plate are for the master cylinder.

It's really hard to find a photographed example of this detail on a 1978 bike to compare with as it's normally covered by the exhaust pipes.

Any assistance in clarifying the above most gratefully received....

Cheers