Thanks for the responses so far. Yea, this one seems a little weird (maybe rare...) since it doesn't say US COAST & GEODETIC SURVEY or the like on it.
m&h - I just thought it may be a property marker, as it's at a distinct corner of the Navy School.
jwahl - I don't believe that it is GU2091, as that is near the Defense Language Institute, and this one is just outside the fence of the Naval Postgraduate School (about 1.25 miles apart). If it is, someone needs to really double check their GPS
But yea, I recall looking at the list of BMs before, and saw an 'M21' elsewhere, but that was in a city 15 miles away, and at a train station, and didn't appear to have 'Naval Department' on it.
BTW, there isn't a map for Bench Marks like there is for the geocaches, is there? That would make this all seem a lot easier. haha