Like the Apache and the Osprey, the Black Ops also had their own, black version. They used one model to transport the nuclear bomb set to destroy Black Mesa.
A bigger truck is featured in the map of5a2, with grey textures. The same truck, this time with the original green textures, is hidden in an out of reach and very dark room, with partially misaligned textures. It was either left there as an Easter egg, since the room cannot be accessed without noclipping, or it is a leftover of an earlier version of the map, forgotten there by the mapper.
Parked truck with less polygons in an early Sector F.