Model Gun Corp Japan

Model Gun Corp Tokyo Japan .177 Pellet Gun

I have a Model Gun Corp .177 Pellet gun purchased in Japan about 25 years ago. It is a S&W replica model 459 On the bottom of the magazine is a receptical to rifill gas. I don't where to obtain this gas any help would be greatly appreciated

( the receptical looks like the type you see on the bottom of a Butane Lighter The gun says LIQUID CHARGING
 
It uses "green gas" I believe a form of propane.....there are thousands of that type of replica gas guns,I have many,all steel and great replicas.
Never seen a pellet one,most are paintball guns, Japan has very strict laws,,,Do post a picture.Go to a paint gun store are online and order it.
Also, try to look it up online....Good Luck