Alan:
I've never seen those postcards before...Here is a random picture I took in 1985 of an Orioles ballgirl spinning a child during the 7th Inning Stretch (can you imagine that being done in 2020??).
I'm thinking that the Miss Orioles' represented the team at local community events or taverns (they were sponsored by Busch Beer) to drum up interest in the club.
As an aside, a couple of those Miss Orioles' married Oriole players (Kathy Lioi at one time was married to Bill Swaggerty & Bonnie Lorber is married to John Habyan).