Part #56 seems to reset the valve knocker or has something to do with it none the least. #57 is allowing the gun to fire full auto IIRC though, but you said that without it it shall still work both fine?
Here's a link for others to the part # in case you could help better than me
http://www.airsplat.com/Manuals/GR-KJW-M4-CARB-7.jpg