On my ex Grads VX1600 car the Webers are attached to the inlet manifold using studs that use nuts on both ends. When I look for images and for parts online I can only find studs with UNC threads on one end and UNF threads on the other end to thread straight into a tapped manifold (I assume). No need for a nut on the manifold side.
Did the manifolds on the VX1600 come with threaded holes on the carb side? If so mine was potentially drilled out.
I am currently a thousand kilometers from my car and won't be back for a month so I can't check if my studs are the UNF/UNC kind.
Reason I'm asking is that one stud was replaced with a counter sunk(!) bolt. I assume that happened when a stud went missing and no correct part was at hand. I would like to correct that as soon as I'm back with the car.