Back when all ARs were .223/5.56, labeling weapons by caliber was unheard of and unnecessary. All that's changed. If you have more than two AR rifles/uppers in different calibers (in my case .223/5.56 and 300BLK), care about your personal safety, and the longevity of your weapons, it's a good idea to ID your uppers/lowers with caliber specific magazine release catches and dust cover/ejection port covers. Safety first!
Very nice attention to detail! 
