US Congress C 1914?

It appears that it may have had a stamp... sorry, I don't know about that.

I'm more curious about the signature, on what appears to be state of MO paperstock. The way I read the signature it would be Ben Humphreys of Mississippi in 1914. Is that what the holes are from a legal file? - current legal plea folders would be more widely placed - or was it a way to cancel the envelope's validity?

A free franking cover that had a stamp affixed over the preprinted signature and then removed later. All members of Congress have this privilege, but the stamp was probably added as it was used for personal usage. I agree, the punched holes are probably used for filing. My stepfather works at the U.S. Post Office of the Senate and sometimes saves me stamps from mail to Congress that's not shredded.
Will

Another possibility is that the holes were used for fumigation. Mail from that era was fumigated to try to prevent the spread of influenza and other diseases. This was long before antibiotics and when child mortality rates were very high. People were, understandably, very worried about spreading illnesses. Holes in the envelopes allowed the sterilizing chemicals to get inside the letter. I've seen some of the covers and they tended to have larger holes than these but the sizes may have varied.

In that case, it's aptly addressed to "Chemical Building".