Ultralinear output stage patent
A lot of what was in the patent description is technical and difficult to understand. Keroes wrote up a short more easily understood description of the ultralinear output stage and its advantages in the pdf file at the link below.
"Theory and Operation of the Ultra-Linear Circuit"
Probably the first commercial amp to use the "Ultralinear circuit" was the Dynaco Mark II which came out in 1955 and was Dynaco's first product. All succeeding Dynaco tube amps like the Dynaco Mark III (1957), Stereo 70 (1959), Dynaco Mark IV (1960), Dynaco ST-35 (1963) and Dynaco SCA-35 (1964) also used the "ultralinear" output stage.