NOS to NOS there won't be any material difference from one mfr to the next. It's a rare day to get a M14 trigger group that's virgin NOS. They got rebuilt, parts got swapped out. I remember trigger groups was one of the very few M14 parts the DCM sold back in the late '80's when they sold a boatload of M1 Garand parts out of Red River Army Depot. We got a few. Each one came in a plastic bag. Sure enough, they had a mix of hammers and safeties, not a nickel's worth of difference from the trigger groups SAI used to install in new M1A's. I don't know how you can look at a trigger-sear, plunger housing, or springs and pins and tell who made them. You just try to get USGI parts from an honest source. At the end of the day a trigger group's value is all in how smoothly it works when assembled and installed in your rifle. I've never run into one that didn't benefit from a little careful tuning.