It is not missing a stamp. It is a type of official sometimes referred to as Penalty Mail. It is used by government officials/departments. The Scott catalog does not list "penalty mail" issues, not even in the US Specialized.
There are probably at least a hundred different penalty mail cards/envelopes. If you specialize in the different departments and different fonts, it can easily get into the thousands of different varieties. In the last quarter of the 1900s, you will often see the Postal Service eagle logo on the card/envelope, along with the "penalty" inscription.
Based on the postmark date, your penalty mail card was issued in the 1940s or earlier and was used by the Library of Congress. I do not know the exact time period of use.
Not very many people who actively specialize in Penalty Mail. I think in the past year, I've only run into 2 that I can recall.
