Looks like someone on the mill machine got a bit rambunctious on slide exterior side. Might of been the slide wasn't locked down in the holder correctly (or cutting chips weren't cleaned out of holder well enough when slide was flipped over to mill) to have one side cut a few thousandths more than the other side. I wouldn't worry much about the interior cut unless its deep enough that you think it will make the metal brittle and crack in that area from usage. You can call MOM and ask about it, along with some pictures to show. They'll probably send you another slide for both conditions. If they do replace the slide, might as well ask them to send out another firing pin and springs for the future need. It'll all be free.