Can you post a larger images of the signature
Maybe someone that is better than myself can comment, but what I look for are the pools of ink that collect at changes of direction. Its hard to tell because its discolored, but I see some evidence at the top of the D and at the bottom of the G. If someone said I had to make a choice, I would say auto-pen, but I would concede to someone that is more familiar with this signature. Not something I deal with everyday.
I asked Herman Darvick to let you know what he thinks. He's quite good on presidents.
In my opinion, your letter was authentically signed by President Eisenhower.