In my Librem 13v3 /sys/class/dmi/id/modalias contains
dmi:bvncoreboot:bvr4.7-Purism-4:bd03/20/2018:svnPurism:pnLibrem13v3:pvr3.0:rvnPurism:rnLibrem13v3:rvr3.0:cvnPurism:ct9:cvr:
thus /etc/udev/hwdb.d/90-keyboard.hwdb contains
evdev:atkbd:dmi:bvn*:bvr*:bd*:svnPurism:pnLibrem13v2* KEYBOARD_KEY_56=backslash