The thread pitch is 12NS-1, NS later being renamed UNS I believe.
The M2 Lock Nut was only used with two types of Fin Assembly:
Fin Assembly, 500lb Size, AN-M109 and AN-M123 series, which fit the Bomb, GP, 500lb AN-M64 series.
Fin Assembly, 1000lb Size, AN-M113 series, which fits the Bomb, GP, 1000lb, AN-M65 series.
From the relative diameter of the overall base plug, I'll lean toward it being from a 500lb bomb casing.
Hope that helps,