The BtoB is 16.5 mm. Below is a link to the track standards that give you back to back plus tyre width as well as track standards
Are the 2 BtoB gauges you refer to the ones you recently purchased from the stores? If so, can you let me know the widths as they should be the same. If there is a problem I will replace them.
Frame widths are generally 13.5mm between the insides. That is the width of the EM spacers. That has probably been based on 15thou frames. Thicker frames may require narrow frame spacers