Facts about the uniform (obtained from GS of California’s Central Coast and the Collector’s Guide):

Intermediate was a category created in the 1930’s for girls 10-13. (Maryellen was nine, but let’s just go with it). With some age adjustment, it would be split in the 1960’s into Juniors and Cadettes.
The Intermediate uniform changed very little from WWII through the mid-1950’s. In the 50’s, short sleeve uniforms started being worn, but long sleeve was still popular. Sashes were introduced around then as well, replacing the old method of sewing patches onto sleeves. There were some examples of orange neckerchiefs, but they were mostly yellow. The AG wiki states that Maryellen’s uniform is a hand-me-down of her sisters but I did not find evidence of that in the books. However, her sister Carolyn would have been her age around ‘49-'50 and her sister Joan during WWII, so they likely did use the same uniform.
Maryellen lived in Florida so her uniform would have been made of a lightweight material. Southern states requested that girls be allowed to wear white for the heat, but only Hawaii could use that color. If a girl owned the long sleeve uniform, it was permissible for her to shorten the sleeves herself.

If you have Seasons, this can be the default Llama Scout uniform for feminine Sims. I’ve made various packages that will override some assets and tag others. It’s a bit complicated so this document explains everything.

Includes two options:

Merged file with the above items for non-scouting use - BGC. You will need the hat slider.
Merged file with the above items plus overrides and extra standalones for scouting - Seasons

The dresses and hat all come in twentiethcenturysims discontinued Khaki Wacky palette with two extra swatches: The traditional bluish-green, and white for Sulani. The three main items are CAS indexed for 1950 but the standalones are not, since you don’t need to select those.

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Maryellen's Intermediate Uniform