The MK II was the standard issue anti-personnel grenade used during World War II. After the war ended surpluses found their way to Empire Bay and into the hands of mobsters and street gangs alike.
The MK II grenade is a single use thrown weapon. The player can carry a total of 6 and it occupies a slot in the left side of the weapons HUD along with other thrown weapons.
They're first used by the player in Chapter 1, when Vito throws one through the window of the town hall while his squad is overtaking it. They're not seen again until Chapter 8, where they can be found near the end of the forge. They finally become available to purchase at Harry's Gun Shop in Chapter 10.
In all three Mafia II DLCs they're available for purchase at Harry's shop from the start.
You can find more real world information about the Mk II grenade on its Wikipedia page.
- The MK II was manufactured with grooves in the cast iron, which gave it the appearance of a pineapple and hence the origin of that nickname.