"Everything You Wanted To Know About Bottom Brackets, But Were Afraid To Ask.
The third table from the bottom of the page is a table detailing the different threading and shell sizes.
The table below that explains the hilarious consequences of fitting the wrong BB to a given shell: fun for all the family!
So, based on Sheldon's page - take spare BSC cup without shoulder, drop into BB shell, if it falls right though it's more than likely to be an Italian shell*. Sounds an easy-peasy way of IDing Italian threading.
*"Oh, and one other thing - in this country they thread their fixed BB cups the wrong way round." </Charlie Croker>
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