The letter that was submitted to the document examiner was a copy, so paper examination(s), such as watermark dating, could not be undertaken, nor were they necessary (for this particular case). And yes, May 7, 1961 fell on a Sunday (and the New York Yankees lost to the Angels by a score of 3 to 5) but that’s not the answer.
The type font is Courier 10 which was in use in 1961, so no help from that corner.
It will help if you are over 60 years old (70 would be even better), so if you’re not, get some help from someone who is.
Use your common sense.