Ron Guth:
The Willow Tree Massachusetts Silver coins were the first attemt at a full design following the simple punches of the New England silver coins. The Willow Tree coinage was produced on crude blanks with a hammer die of some sort. Multiple strikes of the dies, at various angles, were necessary to produce a coin with full designs. However, the multiple strikes often duplicated or obliterated other elements of the design, resulting in illegible, jumbled legends, an odd-spiky-looking tree, dates such as 16522, and/or blank areas on one or both sides. There is no single example that shows the complete design on either side, so it is left to the collector to choose the one that is most "attractive." Certainly, surface quality is an important issue, because many examples are impaired by corrosion, pitting, scratches, or other post-mint damage. Multiple varieties make up this issue, all of which are very rare.
We know of only 33 demonstrably different Willow Tree Shillings.