I agree for the letter dated on Dec 8...
Possibilities without concluding to fast: a doubt in CHIPG assumption1. it's also possible that the letter never got swapped over the years, meaning this letter was effectively in the envelope through all these years
2. also, this letter was mailed from Ste-Catherines, Ontario to what I believe is Wellington (???), Ontario.
When Googled, from Ste-Catherines to Wellington County (somewhere in the County), there are 149 Km (92.5 miles).
Let pretend that the mail delivery was done on a horse, it is possible this letter might have been delivered the same day, something like leaving early in the morning and received sometime on the same day, at the Wellington (??), Ont Post Office
We may also doubt that the letter might have been swapped from an envelope to another. One thing is certain, we have here the same person who wrote the letter and the address on the envelope and a doubt that the letter did not get swapped...OUF

I do believe this letter has always been in this envelope. The envelope was kept
"as is" all this time until...somebody discover it, put it on
ebay, I won it and I share it with you guys
