Just for reference, Burrard gives decimal equivalents for the following vulgar fractions:

16/1 - 0.669"

16 - 0.662"

17/1 - 0.655"

17 - 0.649"

Below is a link to an antiquated post with the German plug gauge values in mm:

https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=213433#Post213433

Difficult to see the decimal values but my wish would be to see an image in better focus when enlarged.

Cheers,

Raimey
rse