Because the bronze carburetors are so uncommon, very few people are aware that they were specially made for the ultra-rare Vincent Bronze Shadow. Unfortunately, it was discovered during testing that the additional weight of the bronze limited the top speed of these machines so they were equipped with 145 mph speedometers.
If one of these speedometers appears on eBay, typically the seller won't know what it is from. But, now you do...
I should add that the Vincent Bronze Shadow was introduced in 1955 to commemorate the 8th wedding anniversary of Queen Elizabeth, to try to exploit the occasion in order to increase sales. However, despite incorrect reasons you may have read in various books, it actually was the development cost of the Bronze Shadow that drove the company into bankruptcy that year.
Although the machine was based on the Series C, Phil Vincent felt the bronze carburetors constituted such a huge improvement that he officially termed it the Series C++ (two '+' because, obviously, there were two carburetors). As evidence of the fame of the super-rare Vincent Bronze Shadow in a small circle of cognoscenti, over two decades later the developers of a computer programming language used the C++ name for a compiler to indicate it was that was vastly improved over the Series C compiler.