The KWA KM4 Magazine Catch Spring is a replacement spring for the mag release on a KWA M4 AEG. Part #173 from KWA, this spring will fit most brand AEG M16s, and is a perfect replacement for the spring you lost because you just didn't know it was there.
KWA KM4 Mag Catch Spring Features:
-KWA M4 Part 173
-Fits all brand AR15 AEGs