The copyright date on the envelope is 1938, the pattern could be a year or two later.
You get to the bush, of course, by flying your plane, perhaps a Stinson SR-10, which was first manufactured in 1938. The film Test Pilot, starring Clark Gable, Myrna Loy, and Spencer Tracy, was released in 1938 and was nominated for an Academy Award (losing to You Can't Take It With You)
This is an unprinted pattern. No fabric recommendations are made, though blue serge was popular. I would guess that dry goods or department stores ordered bolts of blue serge along about August so that girls and their mothers could get their new gymnasium suits made for the beginning of the fall term. My guess is that if the suit was made in the sleeveless version, a white blouse would have been worn underneath.
Evidence suggests that dark bloomers worn with white middy blouses were also referred to as gymnasium suits.
Interestingly, Putnam's Household Handbook by Mae Savell Croy, published in 1916, recommends gymnasium suits as the ideal garment in which to do housework. That may be part of the reason why this pattern was offered in sizes up to a 46 inch bust.