A couple obscure regional special edition Cougars, the Cougar 500 and the Calypso Coupe

A rare regional promotion for the Carolina 500 on June 16 1968.

there were 54 of the Cougar 500s built, but not all were 428's. We know of the existence of one small block Cougar 500 in Tennessee: VIN 8F91C5xxxxx. They were sold in the Atlanta (DSO 21) region and the Washington, D.C. (DSO 17) region.

They were all built between May to July of 1968.

http://www.cougarclub.org/cougar500.htm
http://www.ranchero.us/forum/index.php?threads/special-paint-ford-unknown-70-ranchero-429-cj.11388/

The Cougar GT-E, is rare, only 264 were made in 1968.

The 1968 XR7-G is rare, it was built in only 617 units, and just 14 got the 428 Cobra Jet.

The Eliminator isn't as rare, with more than 4,500 made in 1969 and 1970, but it's quite famous. 

But the rarest version of the Cougar is the regional model that was sold only in Atlanta and Washington DC in 1968 as the Cougar 500. 

It didn't come with notable extras beyond the "500" badges and racing flags on the rear fenders, but it's rarer because Mercury made only 54 of them.

https://www.autoevolution.com/news/1968-mercury-cougar-500-sees-daylight-after-31-years-it-s-rarer-than-hen-s-teeth-193871.html