The setup that Willy's video pictures is what I described in my earlier post. It could well be a 400 series. The vacuum device on the power divider lock. They work much like a transfer case. Instead of having differential action between the two axles like Rockwell and later Eaton as well, it was only locked in for extra traction. I'm not sure that anyone beside GMC and Chev a little later used this setup. Ford and IHC was already offering the Rockwell SLHD which was used into the 80's.
At 22,000, these tandems added very little to weight carrying capacity. 20,000 is now carried on a single axle.
Water weighs about 8 1/3 lbs per gallon, The high sulfur Diesel we had up until a few years ago weighed 7 1/2 lbs to the gallon. The Ultra Low Sulfur we have today weighs about 1/4 lb. less per gallon. Todays single axle tank wagons are built to around 2,400 gallons.
My point is, the tandem at 22k provided very little extra weight carrying capacity. I think the main purpose was the walking beam suspension for rollover protection and the screw for extra traction. This is why I would really be surprised if this thing had a drag axle.
