I had installed Slackware 15.0 several times on my UEFI PC, and have several files remain under /boot that are not part of the Slackware tree.

I re-partitioned in an attempt to delete all files off HD, then ran a fresh install but the files are still there. Most of which are past attempts at customzing ELILO, breaking it, and running ELILOCONFIG, so there are replicate init/efi files under /boot, boot/efi, boot/efi/EFI, then finally /boot/efi/EFI/Slackware where they actually belong

Maybe an MBR partition i cannot wipe clean?

They are probably on a separate partition to your Slackware install.

/boot/efi is usually where Slackware mounts your EFI partition.
Run "lsblk" to check this.

If you are dual-booting, probably best not to completely wipe this partition.
Running "eliloconfig" will only affect the EFI/Slackware/ directory (usually mounted as /boot/efi/EFI/Slackware/ )

I would say just manually remove the files you want to get rid of.
The nuclear option would be to reformat this partition (as FAT32) then remount it on /boot/efi and run "eliloconfig"

Since you have only Slackware installed, did this computer come with a pre-installed OS?
You neglected to post the names of the various files you see? Are these files in the /boot directory and is /boot a separate partition or are these files all on the EFI partition? Formatting the EFI partition should have removed everything. Do you see these files when you are using a terminal or in a file manager or both?