On Wed, 7 Aug 2019 17:13:50 +1000
"Ralph Ronnquist \(rrq\) via Dng" <dng@???> wrote:

...snip......
>
> Maybe there's a spurious back-quote in /etc/default/grub ?

Bingo. Thanks all who replied.

The successive posts jogged my brain to look again and the May-2019
date reminded me that I had actually changed /etc/default/grub to
modify the Grub-Commandline to include some parameters to try and fix
the hardware bug with the AMD B350 motherboard, but apparently
neglected to run the update-grub command.

So when the last update/upgrade sequence relegated a linux-image, a
mismatched quote showed its hand.

Fixed and now I'm back to chasing down HW/memory fault.
A side benefit is that now I have a consistent way of crashing the
system and might finally get to a working solution for this pair of
identical AMD B350 systems.