and I read what ODB said, hold offs. However, the tape and the reference just prior to the M79/M203 to me implied he was talking the sling marking with respect to shooting grenades and said 'hold offs' instead of 'expedient elevation markers for specific ranges' (handy for FPF and several other things, not least a channeling barrage from two or three launchers...).

Proper hold offs, as those you mention, IMO need to be known and remembered -- carried in the mind, not (written?) on tape on the sling. I'd ask what good they are there, particularly at night or in low vis or when one is in a terrible big hurry -- or else...

Or are we all missing something?

Only ODB knows and he must be out and about, having fun somewhere...

You really want to get a deer in the headlights look, ask even experienced people about fire tunnels.