[Bobo W]

The short answer is that the AC "Cobras" had a chassis number prefixed with the letters COB and the Shelby Cobras had CSX other than that they were identical.

Picking up on some of the history in the previous posts this is my understanding but I'm sure more knowledgeable people will be able to correct any errors. Sorry it's a bit OTT but there's a lot to tell.

The background to the Cobra comes from a number of sources firstly Carrol Shelby who was a very successful racing driver forced to quit due to bad health (I think) who was looking for a car into which to put the new engines from Ford so as to race against the Corvette, then there was John Tojeiro who was so impressed with the Vignale Touring Ferrari Barchetta 166 that he made his own special drawing inspiration from this design and then there was Derek Hurlock who owned AC who in turn liked Tojeiro's work so much that he bought the design and that became the AC Ace.

The AC Ace was powered by a variety of engines from Bristol to Ford and was a very pretty 2 seater that also achieved a certain amount of success racing in the states which caught Mr.Shelby's eye. He approached Hurlock with the idea of shoe horning the 260 cu inch engine into the car which became the Cobra Mk1 - the basic shape of which was more or less identical to that of the Ace except the rear arches were slightly widened.

The 260 was followed by the 289 with subtle revisions to the iconic shape from Tojeiro. The main change came with the Mk3 with 427 cu inch engine. Remember the Ace had been originally designed for a 2 litre car and it's suspension / chassis was rather rudimentary so struggled to cope with the extra power. So the Mk3 underwent significant redesign by Shelby and Ford in most areas.

As a footnote the original agreement Shelby had with AC was that they would manufacture the body and chassis, some of which would remain in the UK and be completed by AC and some shipped to the states where the engine would be installed and completed by Shelby. Initially the car in the States was referred to as a Ford Cobra but shortly after became the Shelby Cobra with Ford licencing the name to Shelby. However I don't think they licensed it to AC hence strictly speaking there is no such thing as an AC Cobra it was either an AC 260 / 289 / 427 although looking at all the press reports of the time would have you believe that this wasn't the case.