if I had to guess, I bet it is for a shift, or a special ordered truck, and that was the letter designated by the shift for that piticular truck to be followed down the line.....

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Seeing as there are four 77 D150 440 truck owners here it would be cool to compare the MDH between the trucks.

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Mine says
031821L x
Not sure what the L or X are for?

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Not sure what the L stands for. Will have to do some checking on that.

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