Almost a year late on this one! The B-29 has a "buzz" marking on the aft fuselage BF-870, so I would suggest the serial number is 45-21870. It is interesting to note that wiki and one other site, show BF to be a B-29 from perhaps Boeing Field, although the airplane was manufactured in Wichita.
It is also interesting to note that Joe Baughers site of serial numbers,
http://www.joebaugher.com/usaf_serials/1945.html , shows 45-21867 as the last B-29 delivered. So, to speculate, this B-29 is one left over at Boeing.