Fuzes M55A3, M55A2, M55A1 and M55 are used with HE projectile primarily for high burst ranging.
Fuzes M55 series are identical in every respect with fuze M54 except that booster M21 series is a manufacturing component of the fuze. The only difference between fuzes M55A3 and M55 is in the booster with which each is assembled.
MIL-HDBK-146, Fuze Catalog (1982)
TM 9-1300-203, Artillery Ammunition (1967)
TM 9-1385-51, Ammunition (Conventional) for Explosive Ordnance Disposal (1967)
TM 9-1904, Ammunition Inspection Guide (1944)