throught the years ,,,,,
Brakes (size , material and hyd ratio & amount of boost) , rims - tire size and class ('D' - 'E') , springs , shock absorbers- suspension, cooling system (trans, power-steerng, radiator, ect) , steering gear ratio and amount of assist , axle / brg size , clutch pack/plate numbers and materials in auto-trannys, clutch size and materials (ceramic / asbestos type) and pressure plate type in manuals , tranny first gear ratio and tranny selection.
Engine emissions packages, Catalytic converters options, exhaust size and dual / single exhaust, Carb jetting , Camshaft selections, spark-advances/ vac-advance ignition advance curves. Engine control programs.
Fuel tank size and numbers of tanks.
Just to name a few that have been manufactures options that changed from 3/4 - 1 ton trucks in the past. Not as much of a difference to the mechanic as it was to the Fleet sales / order and parts-counter guy.