If my memory is correct, the Dutch amateur economist Sam de Wolff   
discovered the long wave at the same time as Kondratieff. SdW did not   
publish his findings in a professional journal and was not credited   
("named")  for his work (although Schumpeter apparently did mention   
him in this context), most of which was reported in Dutch and/or   
German.  SdW ascribed cyclicity to solar activity -- which is   
interesting because his own  work in capital theory should have led   
him to a better explanation. Based on his rather peculiar statistical   
decompositions, he predicted the timing of the collapse of 1929 and   
that of the subsequent turning point.  
  
History of Thought is not my field -- the above is written from   
memory only. Apologies for any errors.  
  
Jesse Vorst