The horn pictured on the Contmeporarymakers blog site is a replica Berks County, PA screw tip horn. There are some excellent examples of original horns of this type in Madison Grant's book "Powder Horns and Their Architecture". See pages 68 (bottom), and 69 (top) for a couple of really nice examples of "spotted" Berks horns.
This lead oxide spotting and decoration seems to be found most often on eastern Pennsylvania horns ranging from Philadelphia up the Lehigh Valley to Allentown, and a little further east into most all of Berks County. This style of decoration was popular in the last quarter of the 18th century and most horns found with this method of decoration will be from the 1770 to 1810 period.
The dark reddish brown spots are applied by using a paste made from lead oxide, quick lime and then driven with a strong lye solution. Robert Weil wrote a nice description of the process that I use for these spots in one of the old CLA newsletters.
Art DeCamp