My bike is in pieces currently so all is laid bare...
I think this is the stock front mounting method, seat slides forward over the horizontal pins on the frame.
Previous owner put in these rubber bungs glued on top of adjustable screw mounts off the frame rails, which I think is a reasonable solution.
He chose not to use the boxes provided in the seat pan but there may have been a reason for that as I'm sure this is an aftermarket bit of tinware.
As you can see I have recently carried out some remedial reinforcement works to the aft section, aft' that is Mrs Nails complained of going down in the world during a trial pillion run. Not meant in any way to disparage Mrs Nails silth like form as she's still at her fighting weight
PO also made the raised platform I believe and came up with the extended hex screws which work well as you can easily twiddle by hand and nip up with a ring spanner. Why did he go to the mudguard tho? may be the existing holes were too much temptation but personally I would go to the suspension mounts and I may end up doing just that sometime soon. The guard is reinforced underneath with a double thickness section to be fair. The height also seems a bit excessive and I've never liked the gap above the gaurd when the seat is in place. I've got a new foam and cover coming from Leightons so once that is fitted I think I shall reappraise and adjust accordingly.
As you can see after the back of the seat gave way I added two more bump stops on the top of guard to cope with the occasional passenger. These are non-standard M8 bolts nutted top and bottom with a pair of door stop rubbers hollowed out and glued on. You can see the witness marks on the freshly painted reinforcement.
So mine now has no fewer than 8 supports, which I hope is now sufficient. We are really not huge people either!