The numbers you were given are model numbers. Serial numbers refer to the production number of an individual unit, so try to find one with a tag on it, and alter the number slightly to make yours unique. Since you're numbering the unit ficticiously just to be period-correct, I suppose, the actual serial number won't matter as long as it looks right.
There's really no guarantee the model number will be correct, either, so as long as the unit works, the only thing that's being accomplished is making them appear to be model and year correct for your truck.
Jerry
