Thomason Compound Patent Corkscrews.

Why did Thomason refer to these corkscrews as “compound patent”?  The Thomason IX, compound patent corkscrew was made with either four pillars or two pillars. The frame was often bowed due to use over time. An example of the rather rare two pillar version is shown.

The term compound patent may have been used by Thomason to describe the patent mechanism, that is one with two actions and one where the first action is compounded by a second. However, it appears that there were earlier versions of the Thomason made before the compound patent.

Henshall was the first to describe a corkscrew as being a “compound screw”. As well as the well known patented Henshall T shape corkscrew with a button, there was also another patented version shown in patent No 2061 application of 1795. This was the corkscrew Henshall was describing as being a "compound" type. 

Has anyone any suggestions as to the use of the term "Compound"?