The mounting of vacuum motors -upside down--was a common way of doing it from most of the original manufacturers going way back to the late 60's-70's. Pretty generic back in those days, and very easy to design. But, realize that the (HWE) industry was in its infancy back then.
The reason it is not preferred to do it in this manor is water ingress; liquid sitting in the internal cavity of the impeller(s) and thus eventually making a pathway into the electrical motor. In other words, the water will sit there and not be evacuated. For those of us with experience operating these vacuum motors, we quickly became aware that if and when this situation happens, it will destroy the life of the vac motor period. Further, has anyone ever experience the starting up of their machine on their next job and seeing (dirty) water blowing of of the vacuum motor(s) even before they put the wand to the carpet? A real mess and sometimes hassle!
Fast forward to today, with the advent of greater performance extraction levels from some of the industrial contractor systems, keeping water out of the vacuum motors should be a major focus in terms of prolonging the life of these components.
If I recall, years back we received a letter from Ametek mentioning the fact that the mounting of their vacuum motors in that way (upside down) was not advised.
Hope this answer helps.
Best to all;
The JAGUAR MX-2 ETM @ cross-american.com