Morton Salt Girl Poster (original or print)

Hello! I bought this piece at a garage sale. It’s very curious. It obviously depicts the Morton Salt Girl (1941-55) and is still rich in its color (I’m assuming because it’s been rolled up for so long). At first I believed it to be an advertising print. But, as I looked closer and did some research, I started finding more questions than answers. For one, I haven’t come across a single advertisement poster for Morton Salt. They’re all small print ads out of magazines or newspapers. This is much larger - 35”x37.5”
Secondly, every advertisement I find shows much more than this piece. She is usually accompanied by more rain or a bit of text here and there. This is purely her.
Thirdly, I can tell it is not a modern print as it is not made up of dots. It could be a lithograph. I am not an expert so I cannot identify between lithograph and original art. The paper is textured. It reminds me of the paper we used in art class whenever working with pastels. The back of the paper has some smudges of color (seemingly from chalk pastels). It’s the same colors used in the girl. There is no markings on the paper except for “Morton Salt” penciled on the back.
Can anyone help me? I can’t seem to find anything similar online and am at a total loss. I’m wondering if this needs to be preserved with archival standards or if it’s just junk more or less.