Schmid Tool is the mil-spec.  That's pretty much it.  They are the supplier for all the big guys in the industry and have been doing it for decades.  That "S" forging mark is a guarantee of quality.  

These lower parts kits are 100% Schmid Tool and 100% American made awesomeness.  This is as close as you can get to a true Mil-Spec LPK for an AR15.

This kit contains everything you need to build out your lower receiver except the receiver extension components.

This kit includes the ambi version of their selector.

• Hammer & Spring
• Trigger & Spring
• Disconnect & Spring
• Pivot Pin w/ Spring & Detent
• Takedown Pin w/ Spring & Detent
• Bolt Catch w/ Roll Pin, Spring & Detent
• Magazine Catch w/ Spring & Button
• Buffer Retainer w/ Spring
• Ambi Selector w/ Spring & Detent
• Trigger Guard w/ roll pin
• A2 Grip w/ Screw & Washer
• Retail Packaged From Schmid Tool