The word is not the Spanish AÑO but the Latin ANNO (in the year); the macron on the N indicates that the letter is doubled, likely as a space-saving measure in this case.

Putting a macron over an m or n in lieu of writing two of them was a common feature of the handwriting styles in use in Central and Northern Europe at the time, so it was probably adapted from there.

Hi there -- it looks like your comment was caught in Reddit's spam filter. I manually approved it, but it may have been your photo link -- it's giving me a 400 error, which indicates it may be set to private in your Google settings. You may want to try to upload it to another host, such as imgur, and edit the link.

Thanks!