AngelPhiSig is right... lavalier, then pin, then engagement. Even just giving a girl a lavalier means you're serious. Both happened on my campus, but pinning is extremely rare - usually people just go straight to engagement.
I've seen girls wear their boyfriends' letters after being lavaliered - I guess it depends on the fraternity.
If a GDI's wife may weigh in
my husband did lavalier me... he gave me a little heart charm, and my sorority did a candle-lighting for me. I never had a sorority lavalier so I just wore his charm.