Going from this picture, I think it's based on the early AR-10 design.
This photo is from the first round of testing by the army, the one which recommended the very first changes to the rifle such as, reducing the trigger pull, adding a flash hider, giving it removable hand guards, etc.
I'd wager the entire trigger group is almost identical to the early AR-10s. The first AR-15s borrow heavily from the design of early "Sudanese" AR-10's so, it's not out of the question. The modern design may also have been invented to correct the problems the army found with the trigger during the first round of testing; heavy pull, failure to return and some such.