I found the following info on the Pharmco web site:

  "Anhydrous alcohol literally means no-water alcohol, but in reality
  it means low-water alcohol. This distinction is synonymous with
  '200 proof' alcohol.  Another term for anhydrous is dehydrated ...
  Our anhydrous grades of alcohol are always below 0.3% water,
  typically below 0.2% water, and many grades are certified below
  0.1% water."

In my opinion, anhydrous alcohol is for laboratories; to insist on
extreme purity for shellac solvent sounds like over-restoration.
Denatured alcohol works just fine here in Fort Worth, Texas (usually
low humidity).  It is not 100% purity, but is sufficient (and
"correct") for our purposes.

Even the purest of alcohols will begin absorbing water from the
atmosphere the moment it is exposed.  For that reason it is important
to limit the open time for containers of alcohol you will use with
shellac.

Regards,
Robert Linnstaedt - The Organ Shoppe
Fort Worth, Texas