The OEM discs are 4.5mm thick.
But when I took mine off the machined part where the bobbins are fitted is thinner than the main part of the disc. It is machined out on the rear side.
I should have taken a photo

This allows the bolts to sit further away from the fork legs.
I had a chat in Wollaston's BMW yesterday when trying to find out what I need. He said the set up was different for a couple of years in the early history of the GSA.
I may just go OEM at this rate.
The discs will outlive the bike now. I've covered 162.000 miles on the last 2 sets of discs in the last 20 years