The serial number indicates that your frame was probably built 1957-58.
Lugs are Nervex Pro, a high quality French made pressed-steel lugset used by many english and european frame builders. It will almost certainly be constructed from Reynolds 531, but there were many different types of 531 in those days so it could be plain gauge or double butted.
Good luck with the sale.
1996 TJ Flying Gate, 1987 Rotrax, 1969 curly Hetchins Magnum Bonum, 1956 Bates BAR, 1955 Gillott, 1955 Claud Butler Avant Coureur Special, 1953 Ernie Clements Special Course, 1950 Granby, 1948 FH Grubb lugless, 1938 WJ Hood Palace Special