I think this 3-part system was designed to provide a secure air vent for a shed generator.
The 2 sheet metal frames nest - the smaller one is just under 12" x 20" and about 6" deep. The larger slides over that one, just over 12" x 20" but only 5" deep. The door is 15" by 25". Looks like it could be installed with the hinges hidden, and has a slot ground in for a standard hasp.