Shotgun Boogie Gun Works adjustable trigger kit for competition rifles, model Uberti '73.
Includes trigger and safety bar spring. There are two versions of trigger. Position of version 1 is back, like the original trigger. Version 2 is more forward. Smokestack used this version 2 on his world record, 10 shots in 1.55sec.
When install trigger version 1, I highly recommend to cut the edge on lever (see pictures), to prevent bruising your trigger finger.
While this kit is easily installed, it is not drop-in, it requires fitting by a competent Gunsmith.
See also the article in Cowboy Chronicle October 2016 from Larsen E. Pettifogger. Pages 48-55: