Ken -
To answer your original question, How popular were underhammers?, go back to Herschel Logan's book on Underhammers (yes, the author's name was Logan, not Hogan, which may account for some of the problems experienced in trying to find copies on the Internet).
On pages 74 through 78, Logan gives a list of makers, and the location of their shops. It is well to note that this is a list of guns that Logan personally knew to exist, and should not be viewed as comprehensive. There are some 120 guns listed, by makers in 20 States, with a few ( about 22) in Europe. A glimpse at the maps included show the preponderance of these guns to have been made in the East, say from Ohio to the Atlantic, and from Canada to the Gulf of Mexico. with the greatest density of makers in the northeastern States. It is interesting to note that this is also the general distribution of flint and percussion makers in the same period. One interesting exception is California, which shows 3 makers during this same period.
Bill