The Yorkshire Terrier Club of America (YTCA) has a good history on the Yorkie, there are many misconceptions about the breed, and how it was formed. They say: "One of the troubles in getting the Yorkies origins correct is that each of the three breeds necessary to arrive at the final breed of Yorkshire Terrier are now extinct."
For those interested, here's the link:
History of the Yorkshire Terrier by Joan Gordon
By the way, Yorkies were here way before 1972, was that a misprint?