Finally the car was delivered this morning… right before I had to dash off to Toronto :-(
Found time to carefully clean the serial number plate and, lo and behold, there is a faint and crooked “L” in front. Next we checked with Arch Motors and they have no record of having supplied a replacement chassis — they keep careful records to prevent clones. Also, they said that in the early days all the Caterham reinforcements were available as a kit to upgrade earlier chassises. More checking to do when I get home but things are looking even better!