The FNA-B43 (Fabbrica Nazionale d’Armie) was developed in Brescia, Italy during World War II. The Beretta M38 series of submachine guns were serving very well, but the volume of guns demanded by the war allowed the opportunity for other companies and designs to get a foot in the door, and the FNA-B43 was one of these. It is a sophisticated and effective design, but expensive to manufacture (those two attributes so often go together).
The FNA-B43 is a sturdy and well-constructed gun, made of machined components and not stampings. The gun is designed with ease of carry in mind, with a folding stock and a pivoting magazine well. This allows the gun to be easily transported with a full 40-round magazine in place (it uses standard Beretta M38 magazines) without being bulky or risking accidental discharge. It uses lever-delayed bolt design which is very effective (and fires from the closed position) but time-consuming and expensive to manufacture.
The pair of levers force the rear part of the bolt to move before the front part can, thus delaying extraction of the fired cartridge until after pressure has dropped to a safe level. It also slowed the rate of fire down from that of typical blowback submachine guns to a very controllable 400 rounds per minute.
Roughly 7000 of these guns were built and issued to Italian and German forces before production ceased. They were ultimately just too expensive to justify, which is unfortunate because the design is a good one. We have an extensive set pf photos available here and on the FNA-B43 page of the Vault (note that this particular example has been deactivated by welding over the firing pin hole in the bolt and pinning the barrel).