I thought that the driver version would be the issue (340.102 in Devuan vs 340.107 in PCLOS), so i went about updating the driver the wrong way and that got me a damaged X.

Well ...
That was exactly the problem I was in: stuck in tty1 without much idea as to how to proceed, hence my OP.

I eventually figured it out.

groucho@devuan:~$ dpkg --list | grep -i nvidia

... gave me a screen with any and all the nvidia stuff in my Devuan installation.

Then I made sure that /etc/apt/sources.list would not have ascii contrib and ascii non-free enabled.

[root@devuan groucho]# cat /etc/apt/sources.list
## package repositories

# Changed - 20180619
# deb http://pkgmaster.devuan.org/merged/ ascii main 
# deb http://pkgmaster.devuan.org/merged/ ascii-security main 
# deb http://pkgmaster.devuan.org/merged/ ascii-updates main 
# deb http://pkgmaster.devuan.org/merged/ ascii-backports main 

deb http://deb.devuan.org/merged/ ascii main 
deb http://deb.devuan.org/merged/ ascii-updates main 
deb http://deb.devuan.org/merged/ ascii-security main 
# deb http://deb.devuan.org/merged/ ascii-backports non-free contrib main 

# deb http://deb.devuan.org/devuan/ ascii-proposed main 

# added x nvidia non-free installation
# deb http://deb.devuan.org/merged/ ascii contrib  
# deb http://deb.devuan.org/merged/ ascii non-free  

Once that was done, I did some cleaning up:

[root@devuan groucho]# apt-get clean
[root@devuan groucho]# apt-get autoclean
Reading package lists... Done
Building dependency tree       
Reading state information... Done
[root@devuan groucho]#

[root@devuan groucho]# apt-get purge
Reading package lists... Done
Building dependency tree       
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
[root@devuan groucho]

[root@devuan groucho]# apt-get check                             
Reading package lists... Done
Building dependency tree       
Reading state information... Done
[root@devuan groucho]#

Then I went about uninstalling everything that had an nvidia label.
This is just one of the files:

[root@devuan groucho]# apt-get purge xserver-xorg-video-nvidia

Eventually, the list shown by dpkg --list | grep -i nvidia came up empty.
To be on the safe side, I did apt-get check again and verified that I had not broken anything else.   8^/

I then enabled http://deb.devuan.org/merged/ ascii contrib and http://deb.devuan.org/merged/ ascii non-free in etc/apt/sources.list.
Without that, I would not be able to find the non-free drivers I wanted to install.

As I did not know the exact name/version I would have available, I asked:

[root@devuan groucho]# apt list | grep -i nvidia-legacy-340 | more 

In the list I saw that there was an updated version, albeit not 340.107 (340.106), but closer than the previous 340.102 version.

So I installed it via apt-get install and that was it. The only thing I found odd was that at some point of the installation it listed as recommended nvidia-legacy-340xx-driver-libs-i386 but it was not available in in the repositories.
I thought it was odd because of i386.

On reboot X started as usual but the artifacts I mention in my OP are still there, so I'll have to see if there's some setting in Xorg.conf that may be causing them.

As having http://deb.devuan.org/merged/ ascii contrib and http://deb.devuan.org/merged/ ascii non-free enabled in etc/apt/sources.list as a default option is not a good idea, I commented them out again.

This is how I recall the recovery process, hopefully I have not skipped anything important.
It may be useful for someone who ends up in the same situation.

The lesson I learned:
All -contrib and -non-free stuff should be updated by hand and individually, just like when they were installed.

Cheers,

A.

One might also grep around in /var/log/kern.log to check for kernel level complaints, and maybe /var/log/syslog for user space complaints.

Hope this is helpful.

I have an up-to-date ASCII rig with a pair of Nvidia cards running three monitors on Xinerama.
With the latest kernel update I began to briefly get some artifacts on two of the screens when the desktop started.

Not a big deal but sort of annoying and we all know how this goes ...

So ..
What did I do?

As I did not have these artifacts in my PCLOS rig (exact same machine/hardware, just different drive) I checked the proprietary drivers' versions and sure enough, the PCLOS version is 340.107 and the Devuan version (was) 340.102 and I said to myself: "yep, this is it".

One thing I evidently did not take into account at the time was that my PCLOS setup's kernel (4.14.51):

[groucho@groucho ~]$ uname -a -p
Linux groucho 4.14.51-pclos1 #1 SMP Sat Jun 23 22:41:29 CDT 2018 x86_64 x86_64 x86_64 GNU/Linux
[groucho@groucho ~]$ 

Not the same as Devuan's ...

But I went and upgraded the driver via Synaptic but when the warnings showed up it was too late and I did not know how to back out of the dark alley I had wandered into.   8^/.

The result is that now I cannot start X.

I'd very much appreciate if some kindred soul would give me a hand at straightening this out, preferably ending up with the 340.107 Nvidia non-free drivers or at least the ones I had installed and worked perfectly well.

Thanks in advance,

A.