Oddly enough, cellulose acetate isn't really used anymore. The primary use was photo film. Under certain conditions it can degrade and give off acetic acid as it does. It also shrinks.
Crystal mounts were the thing for the better stamps when I was a kid, but haven't been used for a long time now.
Interesting to see that the national archives used it even knowing it wasn't all that great. It's probable there just wasn't anything better. Polypropylene and Mylar didn't come along until the mid-late50's,
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