Q840av gets a make over

Does the motherboard have a label for the connector in your Power Mac 8600? Is it J20?
In the Power Macintosh 9600 schematics, J20 is called "Product LED" and has 5 pins.
https://www.macdat.net/repair/apple_schematics.html

The five pin power LED header is labeled J38 on my Kansas PM8600.

The LED I'm using does indeed have the loop to tell the board that it's in a tower case. All these years I'd thought it must have been the power supply that provided that information, and couldn't understand why my current "desktop 8600" identifies itself correctly.
 

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The five pin power LED header is labeled J38 on my Kansas PM8600.

The LED I'm using does indeed have the loop to tell the board that it's in a tower case. All these years I'd thought it must have been the power supply that provided that information, and couldn't understand why my current "desktop 8600" identifies itself correctly.

My Power Mac 8600/200 is also J38. My Power Mac 8600/200 does not have either BoxID bits tied to the centre pin (/BoxPD).

In the 9600 schematic, BoxID(1) and BoxID(0) are hard coded as 01 (a pull-down and a pull-up resister). The 9600 schematic shows /BoxPD is not connected to anything - the R121 pulldown resister is marked "NO STUFF".
Power Mac 8600 200 Power LED J38.jpeg
 
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