Is that the South African weapon? The one chambered for 20mm Mauser?
Not Quite. The l/39 *Lahti* Norsupyssy and its German Counterpart, the Solothurn S-18/100, fire the 20mm x 138b round also used by the German FlaK 30 and FlaK 38 Flakvierling quad-mount AA guns. The Finnish use was as an antitank rifle for use against the then-light Soviet Armor of the 1939-40 period, particularly the amphibious variants and armored cars, whose armor skin was thinner. By the 1950s, the guns were hopelessly obsolete against main battle tanks, but still offered useful performance against armored attack aircraft, particularly the fixed-wing Soviet IL-2 Sturmovik of the WWII and postwar period, followed by the rotary-wing Mil Mi-24 and other helicopter gunships. Though as designed the l/39 came with skids very suitable for prone deployment on firm or rocky ground that could also be augmented with skis for firing or transport on snow, it required only the design and adoptation of a vertical *fencepost mount* that allowed high-angle firing at aircraft.
The first selloff of around a thousand antitank Finnish L39s came around 1960, the last bunch of Model 39/44 AA versions disappearing from the reserve stock arsenals and warehouses by the early 1990s, when replaced by ground-to-air, shoulder-fired guided AA missiles.
Sixty years of good service for Finland made the l/39 a pretty good deal for the Finnish taxpayers! There's a little more interesting info on the L/39 and its ammo *here*
