The threads are 1/4-28.

The basic specs of the original GI A2 grip screw are......

SCREW, MACHINE, RIFLE GRIP
NSN: 5305-01-268-1191
Part Number: AN501D416-18

THREAD SERIES DESIGNATOR: UNF
NOMINAL THREAD DIAMETER: 0.250 INCHES
THREAD QUANTITY PER INCH: 28
THREAD LENGTH: 0.992 INCHES MIN AND 1.125 INCHES MAX
FASTENER LENGTH: 1.063 INCHES MIN AND 1.125 INCHES MAX
INTERNAL DRIVE STYLE: B10 SLOT
HEAD STYLE: A33 FILLISTER