Hi Pratty,
Welcome to our friendly forum and I hope it serves you well.
In answer to your question:-
The set up with the angled brackets to each side of the rear guard is correct for 1960-on models. Up to...and including 1959 the seat was fastened to a bracket that went across the top of the rear mudguard. The mudguards were the same except for the different hole positions and the seat didn't attach to the "mounts" in either method.
Either method will do, depending on how close to original you want the bike to be! Does your seat have provisions for attaching by both methods? If not, the facilities on the seat will tell you which method it was made for. If the angled brackets don't match up to holes in your seat base and you want to use this method, you probably have the earlier type seat.
If you have the earlier type seat base the easiest way to resolve the problem would be to obtain (or make) the crossover bracket, drill the top of the mudguard accordingly and put grommets in the original holes.
The other alternative would be to get the correct seat for a 1960-on model.
Can you let us know the outcome please?? I'm sure that this topic will be interesting to some of our members.
Beezageezauk.