The M125 and M106 were variants of the M113 armored personnel carrier that saw wide spread service during the course of the conflict in Vietnam.
The M125 was armed with a M29 81mm mortar while its big brother, the M106 was armed with a M30 4.2in mortar. This allowed both vehicles to provide quick and responsive fire support to the hard slogging grunts during a fire fight. Both variants were also armed with a .50 cal machine-gun in case the enemy got too close.
One advantage the M29 81mm mortar had over its counterpart was the ability to bring fire much closer to the vehicle than the M106. This often could mean the differences when trying to gain fire superiority after an ambush.