rumour has it that it might be an oversize piston - but thats just a guess.
They look the same and measure the same diameter but possibly the number denotes the supplier OR it could be an OEM number. I do know that the spark plugs at Austin Rover Longbridge used to have an OEM number and Joe Public only got these plugs when he bought a brand new car. The idea was that anyone found with these plugs in his ten year old banger would be right in the poo!
