It appears from the pictures above that the Model 58 carrier dog slot design followed through into the early versions of the 870 slide assembly and that Remington has revised it a couple of times to arrive at the current "closed off but cut through" design for the slot.According to the Kuhnhausen manual, 870 slide assemblies have through slots for engaging the carrier dog, while early slide assemblies for the Model 58 had grooves instead. The manual really doesn't go into much detail on this though.
I'm not sure why I find such trivia so fascinating, but I do!