I did the same thing. I painted numbers on the top of each holder, and then notched the flaps, a narrow notch for "1" and a wider notch for "5" in the Roman style.
Combined with a tiny pocket notebook, I can keep precise records of each exposure. I then copy the data on to the margin of the sheets, with a fine point paint marker, so each sheet carries it's own data. These "smartphone" apps for the same thing just make me roll my eyes, what a waste of time and effort. Occam's Razor. My system has worked perfectly for almost 40 years and it's fool proof & non-volatile. The only way to lose the data is to lose or destroy that sheet, plus there's the convenience that the data appears on every contact print, which is the only way I print 8x10.
I also use a machinist's scribe to write on the leader of 35mm film. Roll film requires a bit more care and organisation.