There's even this graphic at the Mu Theta website:
National orgs have pledge or new/associate/probationary member pins, but those are produced on a large scale. Locals often find other ways of identifying pledges. Can't say I've seen beanies since
Animal House, but maybe there's a campus tradition at play here.
Meanwhile, they weren't beanies and they didn't replace probationary member pins, but we all had pledge hats back in the day. I still have mine -- it bears the signatures of all the active brothers when I pledged.