I came across this 1929 letter and scooped it up for ten bucks. The document itself is 9" by 6" and has an original watermark of Hammermill Ledger. I think the letter itself is legitimate, but I'm not sure about the signature. Every other similar letter I've found for sale online is listed as an authentic signature, but I have my doubts that someone would sell a document really signed by Helen Keller for so cheap. I have attached images for you to review. The ink is black while the signature is a dark, greenish color. 

I am sure that the "signature" is original to the document, but I cannot tell if it is actually signed or printed as part of the stationery.

Any help would be great.

Thanks!