The Viper Mk I was an experimental submachine gun developed in the UK for use by military policemen in post-WW@ occupation West Germany. It was a simplified Sten gun (full-auto only, without the semiauto option normally included in the Sten trigger mechanism) put into a wooden housing. It was intended to be carried slung over one shouldered fired under the arm with just one hand. To this end, it had neither sights nor trigger guard. The whole concept seems pretty questionable, and while multiple different Viper submachine guns were designed to fill this role, none were ever adopted.
Many thanks to the Royal Armouries for allowing me to film and disassemble this very rare rifle! The NFC collection there – perhaps the best military small arms collection in Western Europe – is available by appointment to researchers, and you can browse the various Armouries collections online here.