These nails I've seen don't typically have two letters followed by four numbers like other benchmarks, but rather just four numbers. They often do have a trianglulation symbol drawn around them or painted next to them. These nails range from MAG nails to 60D's and others.
Most of them also have a plastic washer labeled "Survey Marker - Do Not Remove". If not, they have Survey ribbons under them that range in color from orange to blue or pink, and sometimes multiple colors.