I believe and someone correct me if I am wrong, it is a old wooden yardage marker
edit....
"A couple of times I ran across these tombstone looking things in the middle of some fairways. I figured they were yardage markers but I was wrong. I stopped to ask a volunteer and he told me that they were field boundary markers. I guess the Old Course was at one time a field and these tombstones marked the boundaries of the field. He also said that on some of the markers one side has a G and the other side and R. The G stands for golf and the R for rabbits. Very interesting."
Cheers