A lot of stocks were refurbed and recycled.. look for excessive sanding along the grooves for your hand on the stock. It will be easy to spot, as the grooves won't look even. Also look to make sure you can't see another number stamped on the stock where the serial number is. Sometimes they didn't sand the old one out all the way and it bleeds through. Check to make sure the gas cut off switch rotates freely.. along with the bore these were often corroded and sometimes stuck in position.
After that I think it comes down to aesthetics. Dot vs Tritium Tube type night sights. I think there's about three different variations of grenade launcher. Probably at least as many wood types as well.