I can see the problem...what if the car were purchased by someone who had no knowledge of the replica industry? That person might well do some (a little) research, discover that COX and COB were legitimate chassis designations used by AC...and believing he possessed an original (innocently, not knowing the 0666 number was not ever assigned to an original) attempt to sell it in the future, mistakenly identifying it as an original AC product.
There COULD be a misrepresentation in that case...but "fraud" to me implies knowledge that the item is not as represented. If those future owners were not astute enough to know the history we do know, misrepresentations could daisy-chain down the line of multiple future owners.
I know, I know...buyer beware...but due diligence would be very different for an uninformed purchaser than it would be for a more knowledgable buyer.
I have no doubt the current owner has no intention of misrepresenting the authenticity...but down the line there could easily be an opportunity for a fraudulent transaction, unbeknownst to the new owner.
Removing those numbers seems a good way to avoid all that.
Cheers!
Dugly