That battery was designed by my late father....
Cool. Have you been there?
5.56mm
So, I was a kid and I think the battery was still in use. He also designed a whole string of Radar stations all up and down the keys. You can recognize them because they look like a huge golf ball sitting on a tee ready to be struck. I don't know if all of them are still in use or not, but I suspect so and used by the DEA to spot drug runners. But that's only a guess.