Much research has revealed that my RPB M11-A1 is in fact of KSC manufacture which has the patent to stamp the full RPB label and Cobray motif. It is similar to the HFC, KWA, ASG and so on. I sourced the HFC M11 magazine that should be compatible which has been ordered and eagerly awaited. Thanks guys for all for your help in this matter.
I have the RPB Industries M11-A1 Airsoft version as I believe was manufactured by RPB themselves and not by any other manufacturer. I do not have the box. The pictures show the markings on the lower receiver unit and the one other one is of a WELL HG203 magazine which looks about right but not knowing if there is any conformity to any standard, it is questionable.