[China] PLA WZ-9 and WZ551

The Chinese PLA inventory has a WZ-9 and a WZ551. The WZ-9 is a helicopter, and the WZ551 is a wheeled 6×6 APC (Armoured Personnel Carrier).

The “WZ” in “WZ-9” is acronym for “Wu zhuang Zhi sheng”, or literally “weapon equipped, vertical ascend”, which means “attack helicopter”.

The “WZ” in “WZ551” logically doesn’t mean “Wu zhuang Zhi sheng”. Does anyone know the “WZ” in “WZ551” is acronym for what?