Pete,
Its c/n is 31T-7620051 (built 1976, series-prefix 20 airframe# 051)
I could not find the original owner data for years 1976 or 1977, 78,79.
Original factory registration: N82048, to N111HH, N631MC, N631WF and current as N788BB
http://www.airport-data.com/aircraft/N788BB.html
Cheers
John