The so-called 'baseball', so named beacuse of its similarities to a common baseball, is an American model of fragmentation grenade. It was originally developed in the late 1950s, but it wasn't used until the Vietnam War in the late 1960s, which would explain its apperance in Vietnam for Rambo: First Blood Part II. After 1969, the M67 was the most commonly issued grenade in the world.
M67 grenades are seen when Rambo invades the Vietnamese prison camp and grabs a bag filled with grenades to use against the Viet Cong soldiers. The grenades seen here have holes drilled in the bottom that are plugged with clay, so they are clearly inert surplus grenades.
En-Joo was seen arming an M67 grenade after pulling it off a dead Tatmadaw soldier and throwing it at some soldiers. Lewis also managed to shoot a dead soldier before he threw an M67 grenade at a missionary, but the grenade unfortunately rolled to En-Joo's feet and detonated, killing him instantly.
The grenade seen here is clearly an M69 dummy grenade. The M69 is the exact same model as the M67, only it is not an actual grenade but rather a military training grenade. This is noted by its blue paint.