Looking at a book that I have that lists AC chassis, all of those chassis' are numbered COB or COX. Sooo, I assume from that that this chassis was not one of the original chassis built by AC, but rather AutoKraft.
So, if a modernized car is built to the exact specifications of an older car, it would be a replica, would it not?
A few links for research:
http://www.acownersclub.co.uk/
http://www.w2ec.com/ACOCindex.html - I got this from here -
http://www.w2ec.com/MKIV.html
http://www.acheritage.co.uk/