A Chinese reader resident in China here.

Precisely, it's not 64 that triggers GFW, it's 6.4 I'm not joking... 64 is a number while 6.4 is a date (June 4th). I'm not aware any content that's blocked because of 64 but sometimes 6.4 does make a page get REST (temp for a couple of minutes) or permanently black listed if the page/site draws attentions from some guy behind GFW.

AFAIK it's rare that guys working for GFW are good enough or responsible enough do fine grained filtering so they brutally block everything contain 6.4 no matter what the context is. This is common in a bureaucratic org or a dictatorial nation. Because power in your hands is not audited by people. So blocking everything 6.4 is safe enough to make the boss happy, why bother working hard? Just block everything.