You could save all round and buy the one on the forum from a looooooongtime member for a great price. If your lucky you may find the rider's backrest that go for about US$400 used these days. The good news is that seat with the hole comes with a rubber bung but works fine without it and is THE most comfortable factory Triumph Roadster or dual headlight seat made and I have owned and tried them all.
This was on a 2012 Roadster that I bought used and promptly replaced with a Corbin. Unknown if it is original, but it's in decent shape. A couple of boot scuffs but no rips or tears. Sadly, I do not have the backrest. Triumph Part # A9700186 $150 USD and I'll split the shipping.
www.r3owners.net
There are a few thread here on the forms posted by me and others on making custom rider's backrests to fit the hole and a number of good to excellent designs exist.
The rubber pads on the dual seat and the seperate pillion seat mark all rear guards eventually with or without the bracket which resembles the separate pillion seat mounting bracket.
As stated buy the one from the forum member on the classifieds and get the bracket with it.
Better seat anyway as you can add a riders backrest when you find one or make one very easily.