I believe 1956 was the first year Chevrolet had the twin screw tandem. The early tandems used a conventional 5 speed trans. followed by a combination auxiliary-power-divider type transmission with two rear drive shaft outputs. One drive shaft went to the front differential and the other went to the rear diff. It was quite complicated with several u-joints and many grease points involved. By 1960 Chevrolet had gone to the tandem setup that is now used, single driveline from back of the transmission through the power divider mounted in the front differential, short drive shaft between the rears and into the rear diff.
Jerry is right about most early tandems being a dead 'tag axle' type setup. Extra axles afforded more load capacity.
James
