JENS   and all others good forum folks.
A most interesting thread indeed.  I find little to fault with all the input but I have a question:  How can one have two or more magnetic poles on the same blade? If you use a file, hammering or other means to magnetize a blade on purpose or otherwise, would you not have just one pole a North and a South pole on the blade.  It is hard for me to figure out a process that would magnetize a blade with more than one North,& South pole without doing so on purpose by someone along the way.  Leaving it in one place even for hundreds of years would still give you just a North and a South pole.
So my take is that a blade with more than one North and one South pole must have been done so on purpose.
Gene
Sorry 
  Fearn   got it right this time.