Chimerax Turret

"The Chimerax is by far the most widespread variant of the Chimera, armed with turret-mounted, quad-linked Autocannons. Its design was intended to serve as a mobile anti-aircraft artillery piece in extreme situations where no other anti-aircraft capability was present that could also carry a squad of Imperial Guardsmen and provide close fire support for them. While the Chimerax's quad-linked Autocannons actually fared poorly against aircraft they proved more than capable of supporting the ferried troops. This variant has been mostly replaced with the much more effective Hydra anti-air tank."

Warhammer 40k Wikia

Hail Hydra- (BLAM)
The Emperor Protects!

It would appear your regiment has recieved some reinstated and refurbished Chimerax Turrets instead of the Hydra Flak Tanks you requested. Instead, these Turrets have been provided for the local tech priets to affix to your current pool of Chimera APC Transports

The model will loosely fit older chimera and has a ring adapter for the current generation.