The MkII has the same mechanism as the original Mk I but the barrel and stock are removable. The magazine housing can also be rotated to close the ejection port.
The Mk III is a cheaper alternative construction technique for factories which found it more convenient though the barrel cannot be removed nor the magazine housing capable of being used as a dust cover.
The Mk V has the same mechanism with a wooden stock, pistol grip and a forward grip. This Mark also had an improved safety wherein the bolt could be locked in the cocked position or locked to the receiver.
There were several other differences. A recent issue of Small Arms Review has an in-depth article on the history of the STen.
