[ Ref. http://www.mmdigest.com/Attachments/17/02/07/170207_065201_extra-holes-in-Ampico-B-trackerbar.jpg 

I've seen something like that on computer punched card readers (yes,
I'm that old!).  If those extra holes are in a channel that is below
the surface of the rest of the bar, and if they have vacuum applied to
them, they may be used to draw the paper tighter to the surface of the
tracker bar.

On large computer tape drives, they would have alternatively had vacuum
applied to draw the tape tight to the tape head, or pressure to float
the tape over the head reduce wear when the tape is being moved at high
speed.

Bill Budenholzer