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Called the "grasshopper" for its behavior on full automatic, 3000+ guns built, only served with the swedish army.
The 20mm Guns
The 20mm m/40 followed the same pattern of long-recoil automatic mechanism as the 25mm and 40mm guns. It was chambered for a unique and quite powerful 20 x 145R cartridge, and could fire at 360 rpm. On a wheeled AA mounting (below left), it weighed 300 kg; on a low tripod for anti-tank use as shown bottom left the weight was 65 kg (the same gun was used in both installations, and could be switched between mountings). In anti-tank form it was reportedly known as the "grasshopper", as it jumped about so much on firing! It was known as the PansarVärnsLuftVärnskanon m/40 (PVLV) which translates as "Anti-tank/anti-aircraft gun". The ammunition feed mechanism consisted of an exposed 25-round rotary magazine above the gun, which in the AT mounting meant that the sights had to be fixed to the side. It appears that these weapons were only used by the Swedish army, with some 2,700 guns being produced. It was also fitted to about forty PB m/31 armoured cars and to fixed AA and "combination" mountings.
The 20mm m/45 and m/49 were quite different, being short-recoil guns chambered for the 20x110 HS 404 cartridge, which was ordered in the 1930s but not accepted for service until after WW2. Their development was prompted by concern about the difficulties in importing weapons in wartime so it was decided to make an indigenous design to meet the needs of the rapidly-growing Swedish Airforce. Size, weight and rate of fire were all quite similar to the Hispano HS 404, and in fact the French gun was also in service with the Swedish air force during the 1940s. The Bofors guns saw service in some of the early Swedish jet strike/ground-attack aircraft; the SAAB A21 and A32A respectively. They may also have armed the Pbv 301 and Pbv 302 IFVs from the late 1950s. The m/49 guns are shown below right.
Looks to be a drive sprocket for an M3 Stuart series
Sorry, but I doubt that. The ones of M3/M5 Stuarts that I've seen are looking more "star shaped" besides that an M3 sprocket has 14 "tooths" why the one in the picture has 16 if I haven't miscounted. Not to talk about that the sprockets for Stuarts like many other tanks aren't made out of one part (the one shown actually also isn't but it doesn't looks like it's ment to be taken apart like other ones where you can replace just the "ring with the teeth" in case something broke). I'd also say it's to large and to solid (like in heavy) to be used on such a small tank. I've already done a short search some time ago and was actually looking at M4s up to M24s and even M41s but that's not it. It's quite possible that it's not even US made and belongs to an other tank. Maybe British?