I recall that there were other dairy delivery services but our neighbohood had just Sealtest. They might have agreed to separate exclusive zones. My job as a lad was to take out the trash and empty milk bottles after dinner and bring in the morning delivery without skimming a bit of cream from the paper lid.
You weren’t far from us, if I recall - just inside the DC line? We were just outside, and a lot of the surrounding MD area was still farmland.