I think that you may have run across a transcription error in the database-something not too uncommon considering the sheer volume of information contained in it, and the fact that it was all entered by hand by dedicated volunteers. The handwritten ledger pages
List this as "Wm Ellery/18/Plain St. Bal.". The 1859 models I've seen listed are normally designated as 3/4 somewhere in this(as in the run from 99,401-100,000), and by the omission of this, it is assumed that it is an 1857 model.