index property
Where this sits in the film's own run of marks.
Assigned by the reader rather than written down, because it is only ever used to say the same thing twice: a reel keyed a hold on phase 12 of the dry pass, and the filming pass counts its own marks to find it. If the two runs ever disagree about the sequence, that is the drift the digest guard is for — not something a stored number would have caught.
Implementation
final int index;