It is a holder for three 5mm leds.
It clips on the printer, exactly in front of the extruder head, right behind the sticker "Do not touch".
The leds are angled by 10 degrees towards the extruder nozzle and 20 degrees to each other.
I would propose using diffusion leds in order to get evenly spread light(I haven't sadly :) ).
Mounts firmly by clipping on the plastic above it.
Has a hole for the wires to come out on the left hand side, very close to where the cable bundle of the printer ends, so very easy to get the power supply through there.
Is rather discreet.
I printed mine using 0.2mm layers.
Will update with further photos.