Never heard of her, but here’s a few thoughts.
The address on her card is given as the Park Central Hotel, but the registered package sent to her is addressed as “Park Sheraton”. The Park Central was bought by Sheraton and renamed to the Park-Sheraton in 1948, so it has to have been sent later than that, but probably not much later.
If the card was printed before 1948 and she was 17 at the time then at least some of her experience must have been as a child performer for her to claim 7 years movie experience and 8 years radio experience. There’s no record of her in IMDb.
Some of that experience must have been in wartime, which ties in with the mention of O.W.I. on both radio and recorded spot commercials, as well as the mention of “Keep ‘em Rolling”. I assume the O.W.I. is the United States Office of War Information, created in June 1942 and in operation until September 1945. They made radio broadcasts, short films and such of a patriotic nature to connect the battlefront to industrial production and civilian communities, using the propaganda catchphrases: “Keep ‘Em Rolling”, “Keep ‘Em Coming” and “Keep ‘Em Shooting,” The “Keep ’em Rolling” phrase had also been used by the Office of Emergency Management from 1941 as America began to ramp up its production prior to entry into the war.
I would also guess that the “Cavalcade” mentioned under her Radio experience relates to the “Cavalcade of America” show aired on radio from 1935 to 1953. It was an anthology series with dramatised true-life stories of a feel-good nature, biographies of popular composers and occasional musical productions, originally on CBS, and sponsored by the DuPont Company. The actress Helen Hayes (also mentioned on the card) featured during the life of the series. The card also mentions “Big Town”, which was an extremely popular serialised radio drama about a crusading newspaper editor that aired from 1937 to 1952.
Looks like your Marilyn had a history of child/teen bit-parts and commercial work during the late 1930s to mid-1940s which may well have gone uncredited, and was looking to audition for bigger roles in musical productions. There’s no mention of that song anywhere, so probably whatever it was written for never happened. Who knows whether she ever made a career for herself. Given she was only in her late teens or possibly very early 20s, she might have married and achieved something under a different name.