Now that I've got a 169, I thought I would put Gee's 1971 observation to the test, and so bought a 1909 F168 penny of equivalent wear, and compared the two fists of Britannia directly together under a loupe. I can confirm that yes, even with worn specimens, there is a definite and discernible difference between reverse D (the F168) and reverse E (the F169). The outline of Britannia's thumb on reverse E is distinctly more rounded towards the base as it leaves the wrist, than her thumb on reverse D, which is altogether straighter in appearance. It was very obvious under magnification, which surprised me, as I wasn't expecting anything more than the most subtle variation. Now that I've confirmed the differences to my own satisfaction, I also note that it's visible to the naked eye.
The helmet plume on reverse E is slightly shorter, but it's not anywhere near as obvious as the two thumbs.