The company was founded in Munich in 1984 by Günther Lutz, and became Germany's main Martin Guitars distributor. 

In 1992 they started making single and tricone resonator Guitars in Munich under the branding of Continental, (I have not seen any Ukuleles under this branding though?), and having classical Guitars made for them in the far east under the name Antonio Hermosa. In 2011 they brought the name and logo rights of Sigma from Martin and started to use this on all of their wooden bodied instruments, including a Soprano, Concert and Tenor Ukuleles which they made to look as Martin like as possible,  (and it appears they stopped using the Antonio Hermosa branding).

Now there was some "fun" with this deal because Martin, who registered and started using the name in 1970 to brand Guitars and Mandolins made for them in the far east, (originally Japan but then moving to South Korea, then Taiwan and finally Indonesia), to compete with people like K Yasuma and Nakanishi. Martin, to my knowledge, never used it to brand Ukuleles and stopped using it altogether in 2007. They also let the US rights to this name lapse and it was picked up by St. Louis Music, so when AMI brought the Sigma licence from Martin they were not allowed to use the name in the US and their products there were, since 2014 branded Kindred Guitars, (though I have not seen the Ukuleles under this branding?) More "fun" here means the US distributor of Kindred split with AMI in 2015 and is now selling them on their own, so I'm not sure about AMI's products in the US at the moment but they are publicly saying they have nothing to do with Kindred any more.

In addition AMI are also the German, if not the European distributor for AXL's Johnson range of resonator instruments, (and presumably the other ones too but it doesn't mention them on the AMI website?)