It's likely a joke, but it sure stands out. Could also be transcription error. The letter itself contains a typo ("if you will privide") which is fixed in the transcription. (It should have been transcribed as is with "[sic]" added to note the original typo.) Too bad there's not an image of the reply.

BTW, the letter was shared to reddit 7 years ago and a redditor replied that it was his uncle Andy:

https://old.reddit.com/r/todayilearned/comments/5a95b8/til_i...

A photo of Andy:

https://imgur.com/gallery/tsWkg

The story has been circulating the Internet at least since 2004:

https://www.archives.gov/publications/prologue/2004/spring/c...