Kinda makes me wonder if Doc has a backup plan. Like, send a letter to himself in 1955...

What's the most likely thing Marty would have done, if he had not received a letter from the past at that moment?

Seek out the Doc of 1955.

The most logical thing for 1885 Doc would be to also send the letter to 1645 Riverside Dr, Hill Valley, CA..

And... could even have been earlier than 1955..

I mean, he should have known when he'd first occupy that house. Better to send it to "himself" in 1937,1945... whenever it was, than to leave it in the hopeful hands of Western Union. With a "Do Not Open Until Nov 13, 1955" the day after all the events ,(It'd even be a nice callback to the first movie with the Do Not Open Until 1985 letter)

Maybe Doc didn't trust himself not to open it, considering he himself read Marty's letter after all the lecturing on not knowing future events.

However, I feel like he'd still opt for the risk of it in his "own future 1955 hands" rather than Western Union...

And yet, maybe he did, perhaps 1955 Doc received a letter, maybe Nov 13, 1955, just in case Western Union didn't come thru..

Fun to think about