An online search for "BB tolerance" or "BB standard" results in many pages and technical drawings showing the tolerances to which bottom brackets are made, and to which bottom bracket shells should be made. One notable missing standard seems to be the good old 1.37" × 24 TPI threading of the BSA/ENG/ISO BB.
I've only been able to find two sources that in any way document the supposed tolerance of a BSA BB shell. One is Campagnolo's (PDF link), which says the shell width ought to be "67.2 mm - 68.8 mm". The other is a vague table on "Wheels Mfg" (website link), which lists a diameter of "33.6 mm - 33.9 mm" for the shell diameter. Where are these numbers taken from? Why don't both sources define both values?
I'm not a mechanical engineer so I've not thought to check any engineering-specific documents where these tolerances might be listed, but I'm willing to bet it is documented somewhere; What are the tolerances to which a threaded BSA BB and shell must conform?