For LM4, KM4, VM4, and RM4 Series upper receivers (delta ring) threading, KWA products will fit airsoft-type common threading, which is 1.25" x 1.5mm. Other examples of manufacturers that use this threading are Classic Army, Tokyo Marui, and several others. KWA upper receivers do not have real-steel type threading.
KWA keeps no information regarding fitment for specific aftermarket brands or other manufacturers, and will not officially recommend a certain modification or "upgrade" over another. We do recommend partnering with the international airsoft community for information regarding those subjects on the KWA Forums at www.forum.kwausa.com.