I think you are correct, Jack. That seems to be the consensus. Apparently there is a valve held closed by a spring at the top. The animal pushes on the plunger (the smiling face 😊) which lets the water in to keep the bowl full.
100% correct . Used them in dairy barns so the cows could drink when they came in to be milked. Couldn't use a bucket or trough, they would get " contaminated" or kicked over. Besides the cows were in stanchions in the barn.
Believe it or not, the cows knew which location to go to in the barn and always used the same slot.