Hi,
In all my years of looking for an -54R front back plate, I have never seen one in the flesh or in photos...
I believe the std K or KH one was used and fitted with wider specific cam and pivot to suit big twin wider shoes.
This is the assembly mounted on my XLRTT, post 65 front brake mounted reverse at the back:
But obviously fitted with the rear -54R backplate which did exist...
The KRTT conical casting resemble the Norton Manx one, this might be what you saw recently, wb56...
Patrick