Hello Trev - after a bit of searching around I see you are right about the cranked bracket for '59. The parts book for the '58 Super Rocket clearly shows the horn assembly with a cranked bracket (19-2035).
In an earlier ('54 - '57) A series parts book, there's a copy of it on the Rocket Men site at...
https://onedrive.live.com/view.aspx?cid=25607C70B0CF572E&resid=25607C70B0CF572E%21565&app=WordPdf&wdo=1the electrical equipment page shows two Altette looking horn assemblies. One with a flat bracket (Plunger, 67-9068) and one with a cranked bracket (Swinging Arm, 42-9045).
My post above on the flat bracket followed a conversation with a well known Welsh restorer of Altette horns specifically in respect of a horn for my real RGS, which I simplistically thought must apply to all A10s. He maintained that, like Gold Stars, the flat bracket is the one to use and duly supplied both restored horn and bracket - and very good they look. Is his assertion correct? I wonder.
Unhelpfully the majority of sales literature photos from way back show the timing side of machines, obscuring the horn detail. Whether BSA stuck rigidly to particular types may, I suspect, have depended upon availability, and fit, on the day of assembly.
I must stop this now, before getting sucked further into concours nit-picking!