This intrigued me enough to poke around. System 6.0.5, has Portable CDEV v1.2. This is the same "version" included in 6.0.8, and 6.0.8L just says "6.0.8L". Further poking around led me to this:
"Version 1.3 of the Macintosh Portable Control Panel is required for the backlit Macintosh Portable with System 6.0.x. It wasn't included with System 6.0.7. If you're using System 6.0.7 or System 7.0.x, you can use the Brightness control panel to adjust the brightness.
Version 1.3 of the Macintosh Portable control panel shipped only with new Macintosh Portable computers, or with the Macintosh Portable Backlight Screen Upgrade," and later via AppleLink. - Apple Support
Portable Control Panel 1.3 can be found at MacGUI...if it works. If not, yell at me. If you get it working, I would appreciate a screenshot of what Portable CDEV 1.3 looks like -- there are
very few screenshots of the Portable CDEV in the wild.
You can grab a SSW6NAD and use Mini vMac to copy over Portable 1.3 if you'd like.