I photographed these M51 Skysweeper units at the Fort Lewis Military Museum. Not sure if there is some difference in models for these two examples.
M51 Skysweeper
Fort Lewis (around) 2004
Fort Lewis 2015
Fort Lewis 2026
M51 Skysweeper
upper vents on front left are closed, while holes are open on the other gun at Fort Lewis on the same day. Also gun elevation difference.