Welin breech block
From Gunsopedia
The Welin breech block is a stepped, interrupted thread breech, invented by Axel Welin in 1889 or 1890. Shortly after, Vickers acquired the British patents.
Though the US Navy was offered the design a year or two later, they declined and the American Bethlehem Steel spent the next five years in trying to circumvent Welin's patent, before having to buy it through Vickers.
The Welin breech was a single motion screw, allowing it to be operated much faster than previous interrupted-thread breeches, and it became very common on Anglo-American large calibre naval artillery and also field artillery above about 4.5 inches (110 mm).
