Others MP40 O ring

[DPatt]

I have the Legends MP40. The original O ring for the cartridge tube broke the first time I tried to load the CO2 cartridges. I've tried other O rings but they either are too large to allow the plug to be screwed in or too small to seal.
Does anyone know the correct size for this O ring and a reliable source for replacements? I really would like to fire this thing some time.

 

[DPatt]

They had to use these for other models. I know that Umarex makes other fully automatic models, this must be a common part. What is the part number and dimensions for this O ring?