I guess it is just a time period thing.
Top to bottom -- 50471 no slots, shipped March 24, 1910; 60726 slots both sides, shipped July 22, 1912; 62126 ejector gun no slots, Shipped August 4, 1913 --
The 1911 A.H. Fox Gun Co. "Campgire" catalogue shows slots --
The 1912 "Campfire" Catalogue uses the same picture, but the text includes the smallbores and we've never seen a smallbore "pin gun".
Some of my Parker Bros. doubles have slots and some don't.