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It seems that there is a confusion about the mark designations for our cars. I believe what the NAMGBR states is proper. They are a very respected authority on MG cars as most of us know. Their direct link with the UK and authorities there also confirm this. Everyone should check out the link below and scroll down to the MG 2006 winners list, click on it and take special note of the Mark designations for the classes and years of MGBs. I think you'll be surprised. You'll also recognize some of the winners. This also proves that some reference books are wrong.
https://www.mgcars.org.uk/mgcouncil/
https://www.mgcars.org.uk/mgcouncil/