I have been in touch with a long term employee of Rieter, who now in retirement manages their museum.
He has checked machines from 1795-1945 and all machines including guns bear the name J.J.Rieter & Cie. in the casting. He has no knowledge of RSB. That is another dead end by the looks of it.
I am in the position of your inquiry regarding old machine tools made by RSB.
I have an old book with the history of Rieter with all the machines manufactured from 1795 to 1945. Rieter has produced all kinds of machines and even guns in the last 219 years. All machines, without an exception, bear the name J.J.Rieter & Cie. in the casting. (Johann Jacob Rieter).
The initials RSB are unknown to me.
I am very sorry not being able to help you with your problem.
I wish you good luck in finding the manufacturer RSB.
Best regards from Switzerland