I think we need to look at these mortar rounds carefully, collective googling effort needed:

[originalpckelly]

These things look like a number of American designs:
M374A3 HE

TM 9-1015-200-10: M374A3
HE cartridge is constructed from pearlitic malleable cast iron and contains about 2.10 pounds (0.95 kg) of composition B explosive. It uses the M205 propelling charge, consisting of four horseshoe-shaped increments fitted around the tail. The cartridge is painted olive drab with yellow markings. It can be fitted with either the M567, M524, or M532-series fuzes.