Just had a look in my bottom shed but no bracket, It runs in my memory that I may have given it to a fellow A10 owner along with some other stuff, not sure my memory is not what it used to be.
Anyway if memory seves the lower part is rectanguler with a horizontal hole through it at right angles to the bike probably around inch or maybe more, the upper part is threaded and goes up through the hole and fastened with washer and nut.
I'm sure someone in here will have a sidecar attatched and supply picture??
All the best - Bill