ZL3 shows up on cars with the 350 or 375 hp 396 engines. The GM documentation on our '69 L78 car shows it. A friends '69 L78 Chevy II also showed it in the GM documentation. It didn't show up for my COPO Chevelle though. About the only thing common between a 350 and 375 hp engine is the open element breather which the 325 hp 396 didn't have. Who knows?