If my calculations are correct, this is the letter received by Marty immediately after seeing the delorean struck by lightning.

The lightning bolt that hit the Delorean caused a Jigowatt overload, shorted out the time circuits,before sending doc back to 1885

70 years 2 months and 12 days later to be exact. Marty received the letter!

This letter is brand new and printed on top quality aged copper parchment paper

The letter is in 3 parts with a matching parchment envelope just like the film.

Also there is the map of the old abandoned mine adjacent to the old Boot Hill cemetery.

When marty and doc find the delorean with the map. Inside is another letter and a schematic drawing explaining how to repair the time circuits with 1955 components