Nick, those letters are "NG", as in National Guard. Your plate is ID'D as Plate 806 in "American Military Belt Plates" by Michael O'Donnell & J. Duncan Campbell (1996).
In this case, as noted above, it was issued to New York troops. New York National Guard troops. It is an enlistedman's plate, circa 1835-1862. It measures 55 x 72 mm. "Worn by the 6th Company of New York City's storied National Guards displaying their "NG" monogram above the company number. Regulations published in 1835, when they were the 27th Regt. of Artillery, prescribed such plates: waist belt to be secured with a brass plate, bearing the cypher of the Corps and the Designation of the company."
Hope this helps. Nice find!
Geologyjohn