A Finnish anti-tank rifle from the 1940s, firing 20mm rounds. The most distinctive parts of the design, visible from even a glance, are the small skis attached to the support legs, and the magazine that sticks straight up from the top of the weapon.

Against later tanks it wasn't so useful, but early on it was moderately effective in disabling Soviet tanks during the Winter War in 1940. As tank armor improved, it was tasked with other jobs, like long range sniping and anti-materiel use, with a fully automatic version developed for anti-aircraft work.

Wikipedia's entry on the Lahti L-39