It's not a pop-up sprinkler. Been trying to find a match for it, as it looks a LOT like some of the lawn sprinkler shut-off valves I've seen out there. Just haven't found an exact match - for either the valve or the tool.
My thinking is that there used to be a pipe attached to the threads on the underside, and inside that pipe was a rod or shaft that went down, inside the pipe, to where the actual shut-off valve was. This would be for areas of moderately cold winters, where it was not normally necessary to drain the sprinkler systems. Nowdays, it's a given - drain the system before winter, or else risk getting fired! ...Anyway, that's my thoughts. But gotta stop here, as I'm burning up too much of my data allowance looking at all these search engine pics.