Read between the lines of the various post's , nobody here will tell you to transfer the number yourself onto the firewall as it could be classed as being illegal and their advice to do so could get them in trouble .
However mini's had the body number stamped on the firewall , or on the radiator panel , depending on the model . So long as the Rta inspector can find the number stamped onto either the radiator panel or on the firewall and it matches the number shown on the build plate and the paperwork you're showing with the car then you'll be right . It's not your fault that Bunnings and various other shop's sell metal stamps that would be similar size .....

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