1971-Mach 1...Cobra?

Hey guys, need some help from the classic section....

My father is going to look at a 1971 Mach 1...now , we know its a 351 but what we can't figure out is...

On the back, on the trunk...it says "Mach 1" in small block letters to the right but across the back of the actual trunk lip it says COBRA, in block letters (more like a decal though) spread across the lip...

Now, most we've seen, they have said MACH 1 spread across that part and no cobra decal.

Now we can't figure out what it is...I'm thinking the owner put that on himself for a look...I dunno...

Was there any special editions that came out? Or anything?

Thanks for the help guys...

So ya, the guy who had it ended up putting that COBRA sticker on the back..
It is a Mach 1-- and its a 351 Cobra Jet...now I thought only the 428's or 9's were cobra jet but apparantly this is a 4valve 351 with low compression...

Anyone know more on these? And maybe what its worth? It's in fair condition, will need paint...not much rust (if any) and drives straight and smooth...he drives it every weekend...