Brass heads that is. Here's an example of one that was removed today. It still functioned, just didn't clear the Augustine very well. This system is all copper and was installed around the early sixties. Has a 2 1/2" main with very large zones operated by 2 1/2" W/M brass valves. Unfortunately its a real money pit for the owners as the trees are winning the underground battle for space. Plus it has the old (buy the book) spacing of 20' head X 17.3' row, so it doesn't like most nozzles. It was before my time, but I believe the fine threads were how you dialed in your height for the heads. Since they didn't pop up very high the height was critical. Some people will have the Lawn boys weed eat around them for clearance, nicknamed lunar landings.