I have an HP Laserjet Pro M15w which works fine on Slackware64 15.0. I noticed that it uses one of the hplip drivers. The needed driver is not present on Slackware64 14.2 which has hplip-3.16.5. I made a note of the ppd for the printer,

and copied that file over to the 14.2 machine.

hplip-3.20.5 needs the python3 v3.7.2 in SlackBuilds as a dependency. Then I just downloaded the Slackware64 15.0 source directory to the 14.2 machine and ran the SlackBuild and upgraded the resulting package. The HP System Tray Service graphical application does not appear to run on 14.2 so I just used the CUPS Web admin page to set up the printer. CUPS 2.1 would not autodetect the printer's driver so I just added the ppd file that I copied using the CUPS 'add ppd file' option. The only other issue was that the version of xpdf in 14.2 does not 'see' printers. I just installed evince from slackbuilds for printing pdf files.

An earlier attempt to just use the ppd without recompiling hplip failed. The printer could be set up in CUPS but printing generated 'filter failed' errors.

Now, was there an easier way to provide the required drivers?

Yes. Upgrade the box running 14.2 to the latest stable release, 15.0.