Is the gun sound and in proof? minimum wall? Grant made various grades of guns although all were made to a very high standard. The value could vary significantly. Of course as a gun dealer I'm sure you are at least somewhat aware of the range in quality within many british makers' offerings. I believe Grant guns are considered to be in the second tier of engilsh gunmakers and do command a premium although quantification of the premium would be difficut based on the information given and would depend on many factors. Rocketman's spread sheet will give you a number but pictures and dimensions of the piece would tell more.
