Within a June 5, 2008 response from the NTSB, it is explained why unique FDR serial numbers are required to obtain correct FDR data readout:
"The exact serial number of the unit delineates manufacturing break points. The serial number is used to identify when a recorder manufacturer switched from a certain memory configuration to another. This information is necessary to perform the correct recovery of the data."
http://www.savefile.com/files/1607005
Within a May 5, 2008 response from the NTSB, it is indicated that FDR part and serial numbers are normally obtained directly from FAA and airline aircraft records:
http://www.savefile.com/files/1607008
Because the FBI was the lead investigative agency regarding the terrorist attacks of September 11, 2001, FAA and airline aircraft records would have presumably been brought under the control of and made available by the FBI.
Within a June 11, 2008 reply from the NTSB, the NTSB indicates that when FDR part and serial numbers are not known, they are not contained within NTSB published reports:
"The Safety Board will include the manufacturer, the part number and the serial number of the recorder in its formal reports (if known)."
http://www.savefile.com/files/1607012
The NTSB reports regarding the FDR's recovered from AA 77 and UA 93 are virtually the only ones during the past 20 years of major U.S. aviation mishaps, within which FDR part and serial numbers were not published (the complete list is provided below) Because each essential FDR part and serial number were not published, they were presumably not made available to the NTSB.
American airlines flight 77 FDR report:
http://www.ntsb.gov/info/AAL77_fdr.pdf
United airlines flight 93 report:
http://www.ntsb.gov/info/UAL93FDR.pdf