Nicolas, your Jumbo Powermatic is a beauty.
Thank you for your review that highlights the mechanical, historical and aestetic interest of this watch as it deserves.
I was wearing my Powermatic recently and it confirmed that the window at 12 is a perfect solution to display the power reserve indicator without overwhelming the dial.
It is also important to notice that Jaeger was already using a similar window indicator for displaying the power reserve on its car clock in the 1920s.
Here is a scan of the patent that was published in 1923 by Jaeger.
Kind regards