New Arisaka AT Grenades
The Type 2 rifle grenade launcher was an attachment for the Japanese Type 38 and Type 99 rifles that allowed them firing special grenades to fight armored targets. The standard grenades used were the 30 mm and 40 mm grenade. As always there were plans to improve the penetration power to counter the better armor of newer US tanks. Two suggested weapons were the Type 2 75 mm HEAT grenade and the Spigot HEAT grenade. They were never used in combat and it's also not documented how much armor they were able to penetrate.
The FHSW devs estimated that they probably were able to penetrate up to 90 mm of armor and because it's a heavy warhead it also does a lot of damage. So with this weapon the Japanese soldiers are now able to even fight heavy US tanks like the M26 Pershing in their defense of the Japanese mainland.
Type 3 AT grenade
An AT weapon which actually saw action in WW2 is the Type 3 AT grenade which had a shaped charge explosive inside. Because the Japanese manufacturing infrastructure was heavily damaged by Allied bombing it was a very simplistic design using expendable materials like wood and cloth. Three different versions existed which differed in the used explosive charge.
The model in FHSW is the Type B with a maximum penetration power of 70 mm. You use it similar to the Russian RPG-43, in that you sneak up on your target and throw the grenade on it. Of course you should aim for the weaker parts like the rear armor to maximize the damage.