To add (or subtract…) to this topic, the first photograph shows the shape of the seat stay brackets on my 1950 C Black Shadow.
The larger hole has a 5/16" nominal diameter the smaller has a ¼". There is no sign of remnant threads inside the ¼" hole on any of the four "ears," but there wouldn't be if it was originally tapped 2BA.
The second photograph shows the outside-facing surfaces of the seat stays.
Those of you who are observant and have calibrated eyeballs will have noticed that both of the off-center holes on the right stay are ¼", while one of the holes on the left stay is smaller, sized for 2BA.
The third photograph shows the inside-facing surfaces.
In this case, two of the off-center holes are sized 2BA.
However, if your eyes are super-calibrated, you will have noticed that the center holes aren't exactly 5/16" (0.3125"), they're slightly smaller at 0.310". Since nominal 5/16" fasteners have shanks that are 0.305"–0.308" they are a nice slip fit, but what the size means is the holes couldn't have been drilled (or reamed) with standard 5/16" tooling. Typically, holes are made with standard-size tooling, and what goes in them is turned to the necessary size to fit, since turning is less expensive than custom tooling. This isn't the case on the Vincent.
The larger hole has a 5/16" nominal diameter the smaller has a ¼". There is no sign of remnant threads inside the ¼" hole on any of the four "ears," but there wouldn't be if it was originally tapped 2BA.
The second photograph shows the outside-facing surfaces of the seat stays.
Those of you who are observant and have calibrated eyeballs will have noticed that both of the off-center holes on the right stay are ¼", while one of the holes on the left stay is smaller, sized for 2BA.
The third photograph shows the inside-facing surfaces.
In this case, two of the off-center holes are sized 2BA.
However, if your eyes are super-calibrated, you will have noticed that the center holes aren't exactly 5/16" (0.3125"), they're slightly smaller at 0.310". Since nominal 5/16" fasteners have shanks that are 0.305"–0.308" they are a nice slip fit, but what the size means is the holes couldn't have been drilled (or reamed) with standard 5/16" tooling. Typically, holes are made with standard-size tooling, and what goes in them is turned to the necessary size to fit, since turning is less expensive than custom tooling. This isn't the case on the Vincent.