This subject has come up before.  The long, narrow slots are for
automatic tracking.  When a perforation consisting of small punches
passes between the narrow slots, no correction is needed.  When a
perforation consisting of larger punches uncovers both slots, likewise
no correction is needed.  However, when either type of perforation
uncovers only one of the slots, the tracking pneumatic will make the
needed correction.

To eliminate the effect of humidity on overall roll width, the tracking
slots are placed only near the center of the scale.  Unfortunately,
extreme tracking error at the beginning of the roll can transpose the
roll up or down a half-tone, causing one of the take-up spool flanges
to virtually destroy the paper edge.  I gather that to prevent this the
range of this type of tracking device would have to be fairly limited.

Jeffrey R. Wood