hi... your nickname sounds, like you´re from bavaria, germany..
kx500 engines: 83-84 kx500 A1, A2---aircooled ,
85 kx500 B1---first watercooled ,
86-88 kx500 B2, C1, D1 ---the 3. engine-generation and in 86 the first powervalve came in ,
89- 2004 kx500 E1- E16---the 4. engine-generation with the smaller cylinder and the newer powervalve.
you could compare the parts and diferent years here :
www.cmsnl.com