There are similar markers along the rail trail going out of Wolfeboro, and they do appear to be mile markers. If you follow the rail trail out of Wolfeboro, just beyond Albee Beach on Lake Wentworth there is a similar marker that has "S 9" on one side (heading toward Sanbornville), and "W 2" on the other (toward Wolfeboro).
Not surprisingly, these numbers are roughly the milage shown on the trail maps, and based on the running I've done there, seem to be reasonably accurate.
http://www.cottonvalley.org/