The letter is unsigned and undated, but the handwriting is that of Will's wife Margaret. It's messier than her usual handwriting. So I imagine that she wrote it not terribly long before she went into labor. Baby Margaret was born on November 7, 1912, so I've given this letter a date of November 6. Will apparently sent it to Eva along with his letter of December 6:
So probably he found it as he was going through Margaret's stuff after her death.
We are certainly rejoicing over you prosperity, and feel that it is just the beginning of greater things to come to you. This suggests to me that Eva had just started teaching at the school in Philadelphia when Margaret wrote this, but I don't know.