That design on it looks very similar to a 2 piece device I once saw used on an AR15. That was many, many years ago. It simply stopped the sear from reengaging, allowing the hammer to cycle continuously until the trigger was actually released.
Not sure if it made any difference but the lower was milspec. Milled out, not like the semi only ones now. I recall it was an early Colt that didn't have a forward assist. I know early Air Force ARs were like that.
Those are Lightning Links, now classified as NFA devices. There are still some transferrable ones out there but they've now all been serialized and taxed by an ATF Form 1.
That was all it took, provided you had an older Colt SP1 lower and the time to tinker with the timing, to make an AR-15 rock-n-roll.
From the Vault: The Incredible Lightning Link
https://www.youtube.com/watch?v=3ADo_pOKUBY