Some US GI cartridge cases are run on SCAMP (Small Caliber Ammunition Modernization Program) lines. This is HIGH speed cartridge case making equipment. Several lines of SCAMP machinery may be used to run cases througout the year, or on the same lot/year of ammo. Those dots identify which SCAMP line the case was made on.
Any problem with an individual case could then be traced back to the acutal production equip used to make that run.