Lens Rentals are famous not only for their product and service delivery, but their ability to show the insides of lenses and cameras. They often post very detailed descriptions of the anatomy of such devices and in this case they did a tear-down of both the mark II and mark III versions of the highly-regarded Canon EF L 70-200 IS USM.
https://www.dpreview.com/articles/95...ii-and-mark-ii
As a result they say keep your Mk II or buy one if you can get a good price. I had already decided to keep my mark II...