Tanie Militaria got the Specna Arms M4 FLEX (SA-F03) AEG available at their store in Poland and here is what they say... "Specna Arms presents a line of replicas -  FLEX™ . An attractive price  combined with  an innovative spring replacement system  and  high-quality materials  make this series particularly attractive for beginner airsoft players.   

The body, the slide with the  enlarged SF type stock that houses the battery , the front in the M-LOK standard  and the pistol grip are made of polymer plastic reinforced with nylon fiber. Other components such as the barrel, flash hider and enlarged charging handle are made of metal (all screws, pins, shell ejection window and dummy bolt are made of steel). There are markings and serial number on the body."