The copyright of the toy dates from 1974, when he was part of a revolutionary new toy line developed by a Japanese company named Takara, called "Micro Change."  He was only one of quite a few "role play" transforming robot toys known as the Gun Robo series; his brethren included a Browning 1910 and a Smith & Wesson .44 Magnum revolver.  The Walther P-38 Gun Robo itself was produced in both a standard P-38 form and an expanded "MC-13 Walther P-38 Gun Robo U.N.C.L.E. Special" form - the Special had the 3 attachments, although it wasn't particularly show-accurate.  The toy was able to fire small plastic pellets, although since it was a transforming toy with a spring-loaded missile launcher, the action was nothing like that of a real semiautomatic pistol.  The toy became legendary in the US because shortly after it was introduced, US toy laws changed, making it illegal to manufacture such a realistic-looking toy gun in America and thus making the Walther P-38 Megatron hard to find and very prized by American collectors.  Recently, a new Walther P-38 Megatron toy has been created.  Known as "MP-05 Masterpiece Megatron," this toy attempts to create a toy version of the villain that looks much closer to his animated character design, while still being able to transform into a realistic (if oversized) Walther P-38.  It retains the scope from the U.N.C.L.E. Special, since the scope was the character's weapon in his robot form, but it does not have the silencer or the stock.

Thanks to M. Lu for the text.