Have I missed anything?
Yep. I learned that Slackware does not have e2label, but does have tune2fs.
By trial-and-error I learned that the label name is double quoted so I used
tune2fs -L "label" device-name
and added lables to the two data storage drives in the Probox.
Also learned that when viewing block storage devices using blkid the listing
for these two partitions begin with LABEL="name" while the last two drives
had PARTLABELs apparently set when I partitioned them further in the listing.
And I learned a lot of information about each drive using tune2fs -l
<drive_name>. Interesting stuff.
Now in /etc/fstab the Probox drives are displayed with their LABEL or
PARTLABEL names rather than the device name.
All four drives are mounted and each contains what it should.
Whew! Time for another mug of coffee.
Rich