I found it on Irish Pipes list for Montana State. It opened in 1923.
I was asking because I bought this pin. It will be here tomorrow! The pin number is Sigma Omega 62. So I'm guessing it's between 1923 and 1933, and probably closer to 1923.
National probably has the record of who wore this pin, and what year she was initiated. I bought one from Lambda chapter from 1932. Love wearing my antique pin to AC meetings and convention.