The image is a great help, thanks. I see your point about after-the-fact verification by matching the neg to the flap notches. Have to give this some thought.
Using the side rails, one has a bit more room to work with. If I went that way, I'm thinking left side to indicate the side of the holder and the right side to indicate the number associated with the holder, assuming an external label also would go like 1-1, 1-2...10-1, 10-2 or something like that.
Normally, since I still have my training wheels on, I'm usually working with a limited number of negatives and it's not too hard to keep track. However, only a few weeks ago we were on vacation and only took a few loaded holders to do some LF "practicing." Turned out there were lots of nice photo ops so it was more than practice. Fortunately, at the last minute while packing the car, I grabbed the dark bag, a bunch of empty holders and film... just in case.

Ended up shooting more than I had planned and keeping track, despite note taking (need to work on that too), I was a little lost later on.
