The Interdynamic KG9 pistol was made in Miami Florida for a short time in the 1981-82 timeframe by Interdynamic of America, Inc. ATF decided in January 1982 that the gun was too easy to convert into a machine gun, and therefore decided that an unmodified open bolt semi-auto KG-9 was a machine gun. ATF exempted unmodified semi-auto KG-9 pistols made before this ruling from the scope of the ruling, so there are so-called grandfathered KG-9 pistols available in the USA. The KG-9 was based on a Swedish designed machine pistol, the MP-9. After the ATF ruling the gun was redesigned into a closed bolt, striker fired pistol, the Interdynamic KG-99, which was sold in the 1982-83 timeframe. After that Interdynamic went out of buisness.
Variants on this design were also sold as the Tec-9, Tec-DC9 and AB10 by Navegar, Inc., d/b/a Intratec. Intratec went out of business in about 2000, after the 1994 assault weapon ban law banned one of it most lucrative product, the TEC-9 style gun. They were also targeted with liability lawsuits over criminal use of their guns by gun prohibitionists. All of these proved to be too much for the company to deal with. At this point the design is no longer in production.
In the above letter ATF advises the letter writer that bolts for repair of a KG-9 are not regulated as a machine gun, but in ATF's opinion bolts for converting a KG-99 into a machine gun are a machine gun in themselves (as a conversion part) as are a combination of a KG-99 pistol and a KG-9 bolt, even if unassembled.
Above is a picture of the MP-9, the factory machine gun version of the KG-99. My best information is that just over 30 were made in the USA. Unlike the KG-9, this gun has the serial number stamped into a metal plate in the trigger frame, like a Glock pistol, or like the KG-99.
ATF takes the position that attaching the forward pistol grip to a KG-9 or any of the other semi-auto versions of this design makes it into an Any Other Weapon, an AOW. That would not apply to the MP-9, as it is a machine gun, not a pistol. The KG-99 pistol was first sold with the second forward pistol grip, but at this point it is not advisable to have two together.
Above is an exploded view of an MP-9. This one, unlike the one above, has the collapsing buttstock. Not all of the MP-9 guns made in the USA have this stock. These guns also had a select fire feature which not all US made guns have.
Above is another MP-9 picture.
Above is another Mp-9 picture.
The below photos show a KG-9 "replacement" bolt advertised on gunbroker.