I recently purchased a house that has an older sprinkler system. It consists of full circle brass heads that sit flush with the grass and pop up approx 2 inches and spin. The nozzle portion has 2 holes that spray the water out as the sprinkler spins. I cant make the name out on top of the head. Problem is a few of these heads dont spin anymore. Do these heads still exist?

I attempted to swap out with a newer plastic spray head but they dont throw the water as far. The link below is what the top of the head looks like, however, mine pops up and spins.

http://images.hardwareandtools.com/P/u324673.jpg

Any help would be appreciated...thanks.