Hi Peter,
The linked drawing may not be 100% accurate, My books were out in my workshops when I replied and it was blowing a storm, so I did not venture out to search them
Earlier seats had a metal channel where a block of rubber sat into to add support
To support an alloy or other mudguard I made up a "bridge" piece
Two vertical flat pieces bolt to the top mount of the rear stay
a shaped arched piece with two levelled out ends fits over the mudguard and takes the seat bolts
similar to the original 42-9196 but stronger and simpler shape
The vertical supports are welded to the arch,
The arch being 3/16ths thick has countersunk holes so the mudguard bolts up to it
This way no weight is placed on the mudguard
John