SlackBuildsOrg/libraries/pam_usb

pam_usb (Hardware Auth via Memory Sticks)

This is a PAM module for Linux authentication using USB memory sticks,
SD cards, external SSD etc. Simply speaking, it lets you log onto
your Linux user without typing password if the pre-configured memory
stick is detected.

See https://github.com/mcdope/pam_usb for configuration instruction.

In Slackware 15, the right file to modify are
/etc/pam.d/system-auth
/etc/security/pam_usb.conf