I think that when George says castanets he means it should be played in a fast Spanish style as if there were castanets.The Beatles had a kind of short-hand between them that meant certain things . I don't think he really meant he wanted castanets on the song.
Oh I see... Then 'Castanets' in this case means that the song should be performed fast...., doesn't it??
Thank you very much for your suggestion!!
Any thoughts about the other takes??
Best!!
Xose