Lines Brothers was a company in the UK that made sheet metal childrens' toys prior to the war. When production of the Sten guns began, Lines Bros was a parts subcontractor. Their engineers analyzed the design alongside the machinery the company had available and redesigned a version of the Sten that they could make very quickly and cheaply in-house, by replacing the tube receiver with a rolled and spot-welded piece of sheet steel. Their first order came in January 1942, to a whopping 500,000 guns, which were designated the MkIII.

The Sten MkII and MkIII were produced simultaneously, and Lines Brothers was the only producer of the MkIII. Ultimately they got three contracts, although the second one was cancelled before it was completed and the third was never begun. A total of 876,794 MkIII Stens were made by September 1943. Once submachine gun production caught up with British needs, the MkII was found to be the superior of the two designs and only it remained in production.

Many thanks to the Royal Armouries for allowing me to film and disassemble these submachine guns! The NFC collection there - perhaps the best military small arms collection in Western Europe - is available by appointment to researchers:

https://royalarmouries.org/research/national-firearms-centre/

You can browse the various Armouries collections online here:

https://royalarmouries.org/collection/