Sheet Plate Number?

I have 1985 $1 32 subject sheets with a small six digit number on the top
right and left margins and on the back in the same place. Is this number
called a Master Plate Number? This number is not present on modern day 32
subject sheets you can buy from the BEP store. Is this Master Plate Number
still used but moved farther out on the sheet and then trimmed off or has
the use of a master plate number been abandoned? Any information on the
history of this number, like a plate number for stamps, is appreciated.