Steel Frame
The steel frame that extended to the top of the tank was eliminated on the 21 series (1984), presumably because by that point a full insulation blanket for the tank trumped the ability to hold up a counter.
Interestingly, the "short frame" design of the 21-23 models is still much heavier and much more rigid than any of its contemporairies and most modern machines. If you raise one leveling leg of a 21-23, the machine will rock back and forth, not settle out-of-square like most new machines seem to.
T.