Ah, I wonder... if I moved some pieces from my E: (system) drive...?
That's it.
I had
moved various Adobe (& other) .msp files from E:\WINDOWS\Installer, offline.
So now, I (randomly) selected the last few (Adobe .msp) & moved them back.
On doing so, the install installed.
Guessing it needed (at least) the last (maybe two) .msp updates for the current to install.