[xb-60]

Hello Harpoon.

Easy explanation. According to the World Registry, UK Cobras had the prefix COB for "CObra Britain"
and COX cars were "CObra eXport", covering left hand drive cars for sale in Europe.
The four digit number that followed both the COB and COX prefixes numbered from 6001 through to 6062.

I believe that the prefix "CSX" comes from "Carroll Shelby Experimental", not "export".
Arguably, the "C" in COB and COX could refer to those English/Continental COB/COX cars being the "C" iteration of the AC Ace.

Cheers!
Glen