Nice find! Not sure how rare it is. I have a box of 4x5 Kodachrome sheet film with a 1949 expiration date, which I paid $5 for when a local camera store closed at the end of 2008. Even if there was a possibility of using a seventy-year-old film and getting something, I don't believe your film could be processed, as it's not even the same process as the current K-14 Kodachrome process.