With all the Birmingham marks, I'd pen the Birmingham proofhouse and see how tight-lipped they are. I don't consider the "16.3" & "16.4" marks of British origin but rather Belgian, Bohemian or French(did England source many French tubes?) and from the 1870s. For now I would say the difference in measurement is due to the lack of standardization of the meter which occured circa 1875:
"Attempts were in vain in that Belgium claimed its independence from the Netherlands, but the metric system survived and began a slow but steady conquest of the world. In 1866 the U.S. passed a law making the metric system legal.[3]
On May 20, 1875 an international treaty known as the Convention du Mètre (Metre Convention) was signed by 17 states. This treaty established the following organisations to conduct international activities relating to a uniform system for measurements:"
from:
http://en.wikipedia.org/wiki/Metric_system#History .
I've seen other Bohemian examples but the actual diameters were smaller than the actual measurements. The topic of discussion actually is a compression of time and the time segments have to be extracted and viewed independently.
Kind Regards,
Raimey
rse
