3 LEDs, 3 ~1K resistors. Very thin cables. Soarer's doc has details.
phosphorglow presumably did this mod by gluing the cables to the inside of the barrel plate (seems to be his
usual modus operandi), so if you want to stay close to that / do not want visible cables, a bolt mod would be necessary.
[Edit:] Actually, looking at
this SSK mod, he might also have used SMD LEDs and routed the cabling from the front side. That would be non-destructive and reversible.