The first was one with a tandem-charge that was designed to defeat reactive armor, the second was the top attack version.
The NATO definition extends to "all crew-portable direct fire weapons of a caliber less than 50 mm and will include a secondary capability to defeat light armor and helicopters."
Because they were obsolescent from their introduction, incapable of defeating foreign armor, and outclassed by newer German tanks, the Panzer I chassis were increasingly repurposed as tank destroyers and other variants.
The M371A1 round utilizes a special fin-stabilized projectile which employs the shaped charge principle to defeat armor.
It was originally intended to be the medium-range portion of a short, medium, long-range system for armored fighting vehicles in the 1960s and '70s to defeat future armor without an excessively large gun.
RS-132 could defeat medium German armor with a direct hit but caused almost no damage to light or medium armor with a near-miss.
The resulting narrow jet of metal can defeat armor several hundred milimeters of RHA equivalent, such as that used in light and medium armored vehicles.
In the Battle off Samar, destroyer-mounted torpedoes of the American task force "Taffy 3" showed effectiveness at defeating armor.
The PG-29V round has a tandem-charge HEAT warhead for defeating explosive reactive armor (ERA).
The application of shaped explosive charges to defeat armor emerged in murky circumstances in the years prior to WWII.