Lonnie, & Bill, Starting with the 1958 model year, Chevrolet dropped, one of the numerals, in the series numbers. Prior to '58, series numbers in the, Medium, & Heavy Duty Line were, 4000, 5000,& 6000 Medium Duty. 7000, 8000, 9000,& 10000 Heavy Duty series trucks. From 1958 on, they were, 40 Viking, 50 Viking, 60 Viking Medium Duty trucks. 70 Spartan, 80 Spartan, 90 Spartan ,& 100 Spartan series, Heavy Duty trucks.
