This is a Elftmann Tactical Trigger known as the Straight, model AR-9-S.
The ELF AR-9 Drop-In Match Trigger has been built for performance. The amazingly short take-up" glass-rod crisp break and next to zero over travel can be compared to the finest custom 1911 triggers. Adjustability puts you in charge based on your current use. If you are looking for the finest adjustable trigger for your AR-9 / AR-45 platform" this is the trigger system for you.
Features:
Compatible with Sig MPX
Absolutely Drop-Safe regardless of pull-weight
Adjustable from 4lbs to 2 3/4lbs without removing from lower