Hi
I feel your pain. I have been trying to solve same problem for past 3 months
In short, I was informed I needed the SRA5127 but it does not fit my 2019F750GS and instructions, like yours are unhelpful. I was then informed by Givi UK that it was the SR5127 plus an M5 plate. But when Iwent to my local dealer, the parts guy, who is usually excellent, said that the SRA5127 was the correct rack. Duh! I am trying to continue to use my Givi topbox that worked brilliantly on my previous bike and not have to shell out more cash for a BMW topbox and fitting kit. How did you resolve the issue? I am new to BMW family. Thanks.