I'll write myself a note today (about the way to sharpen a file with battery acid) and look. Sorry I've been so sidetracked with trying to get my kitchen done, (in the midst of a total remodel ).
A board can be your friend in the woods. I was using this over this past weekend while dragging downed ash tops out of the woods. I jumped the creek once and decided once was enough, sure is nicer to just walk across it than to jump.
Here it is I finally remembered and dug out the book and took some pictures of the way to sharpen files and some other info! This is also a picture of the book that I got the info out of, it has ALOT OF GREAT INFO! It's alot of old school techniques from the turn of the century! fishingpol you may be interested in this.