Austrian Post In Mexico, 1867?

First of all: sorry for my terrible english :)

I found an interesting letter in my granddad's collection: an austrian officer send from El Potrero to Cordoba, Mexico. Ex offo letter, the date is 1867.01.31. Interesting historical period, months prior to the execution of Maximilian. I tried to find a similar letter on the Net, but nothing... I attach a picture, please help me, I interested in any information :)