It's a simple file, record, or exhibit number, indicating which file the photo belongs to, and where the file is located (FBI headquarters in Washington DC, i.e. Langley). Nothing special about it.
A quick check for copies of FBI background check responses and FOIA replies does not show this format of a record number being used. Then again. many FBI records for paranormal or extraterrestrial events from the 90s would still be classified and not responsive to FOIA requests, so umm, maybe it's a real exhibit number, we just don't have security clearance to see it.