Saarinen 151 by Knoll or an old fake  

Hi there,
I bought a Saarinen 150 armchair but I am not sure if it is original. I have compared it to my original Knoll Saarinen 151 chair that swivels and is upholstered and the proportions/curvature seem to be the same. However, the bottom of the base is cast in a different way and not marked and the screwed on plate underneath the cushion seems to have been replaced at a later stage. It has some age to it and has a couple of knocks in the shell and also the coating seems to come off at some places. Could it be that an original Knoll was repaired at one stage? I don't think it is an Arkana one. Any help, references and tips are welcome.
http://www.flickr.com/photos/42421346@N03/