I was mulling this over on the Stinfo exchange, but they are passing unto history now so I thought I'd put it here too.
As Amals made carbs, they were almost certainly supplying a standard carb that was already in production and jetting & sliding it to suit the ST's performance curve.
ST's on the other hand must have had a good reason to go to the not inconsiderable trouble of designing & making a carb of their own. It was, after all, a pretty unusual thing to do!
Any thoughts gentlemen?
Regards,
Kim Siddorn