User blog comment:Hayashi H/Spring 2016 Event Guide/@comment-25861293-20160513191940/@comment-25630319-20160513202009

Here almost 700 nodes (~350 sorties on easy) 2x U and 1x 401, making the drop rate 0.3% and 0.14%(Poi shows 0.7% and 0.6%) which is ridiculously low. I got them very early in the sorties and since then about 600+ nodes they never showed up again.

I had 1x U and 1x 401 prior farming.