Brass, but the wall thickness is so thin that it simply snapped at the thinnest part at the root of the thread, where the wall had the least area in the hole. Basically they are too thin in the wall, and need to be made larger in diameter, along with making the inner hole the correct diameter to fit the standard socket.
Basically the manufacturer took too much brass off, to cut the cost of the material used in the individual item. They probably charged Siglent the cost of the length of brass used to machine the post, adding the cost of the plating as well. The manufacturers gross profit comes from the removed brass as scrap metal to be remelted, probably around $7 per kilo.