The Stopper is a simple 37mm single shot riot control gun designed by Andries Piek in 1980. The South African police services were at that time using 37mm guns made by Federal Labs in the US, dating back to the 1930s, and the international embargo on South Africa made it impossible to get parts and do basic maintenance on those arms. So Piek (whose other work included the BXP carbine/SMG and design improvements to the LDP/Kommando) whipped out the Stopper in all of two weeks to provide a new domestic-production 37mm weapon for the police.
The Stopper is a simple break-action gun, with a manually cocked, single action, hammer-fired trigger mechanism. Two versions were made, one with the front grip and one without, and all were fitted with collapsing stocks. Production began in 1982 and ran until 1999, by Mitco Special Products under the Milkor name.
As an interesting postscript, Piek was inspired by seeing Christopher Walken using a Mannville 25mm revolving gas gun in the movie Dogs of War to make something similar in 37mm or 40mm. The gun he designed to this end became the Milkor MGL, adopted by South Africa in 1983 and by the US Marine Corps in 2005.