Both cars were built for or by specific Chevy dealers. The yellow car you see here was built by Yenko Chevrolet in Pennsylvania and is number 4 of 201. When the first-gen Camaro debuted, GM had a policy that engines were limited to a maximum of 400 cubic inches, helping preserve the performance of the larger Chevy Chevelle. This hampered the car's performance relative to competitors like the Ford Mustang and Plymouth Barracuda.
Yenko worked around this by modifying the cars at the dealership itself. Don Yenko, the dealer's namesake, ordered the engines directly from Chevy using the Central Office Production Order, or COPO for short. The engine has a high-lift cam, solid lifters and a Holley carburetor. The cars were fitted with power steering, power disc brakes and a 4.10 rear end with Positraction. Visual upgrades include black stripes, a black vinyl top and Endura bumpers.
Like the Yenko, the ZL1 was fitted with a 427 cubic-inch big block thanks to the COPO program. The aluminum block and heads made it far lighter than other big blocks, one of the biggest downsides of the larger engines. Like the Yenko, the car is fitted with a 4.10 Positraction rear end and power disc brakes.
There are no estimates on the Mecum page, but considering the rarity and condition of the pair, expect final bidding to easily exceed six figures.