I maybe able to shed a little more light on the method Harold used to apply the chassis numbers. When I visited the factory and met Harold, I ordered my MK2 that same day and paid Harold the £200 deposit. He then reached for his order book and wrote in my details as well as a chassis number, ( I think every car he sold was in this book ) he then proceeded to write out my receipt including the chassis number which I have shown below.
67 by
Roy Forrester, on Flickr
Provided Harold used this same system from the beginning it is my belief that the chassis Numbers relate to the order in which customers purchased their Midas ( so chassis No1 was for the very first customer ) and not necessary what order it came out the mold and I think Harold used the chassis number also as the customer reference number.
This could be confirmed if some original owners of early MK1s had still got there deposit receipts that shows the chassis number on them.