calSelector delivers for LSS data only those flux standard stars that were observed at the same position on the detector as your long-slit spectrum to ensure the same wavelength coverage. In earlier times this rule was not followed strictly and sometimes the standard star was observed at the center of the field-of-view instead. You can look for observations at the instrument specific query forms for FORS2 and FORS1 by setting the following constraints:
Start The date of your observations - 5 days
End The date of your observations + 5 days
DPR.CATG CALIB
INS.MODE MOS
INS.FILT1.NAME the order-separating filter of your observations (use Any if you had no filter)
INS.GRIS1.NAME The grism of your observations

and selecting for display in addition

DP.ID
OBS.TARG.NAME
DPR.TYPE to identify the standard star (STD) and its calibrations (FLAT,LAMP and WAVE,LAMP)
DET.CHIP1.ID to identify the correct chip for mosaic data
SEQ.SPEC.TARG position at which a MOS standard star has been taken

For FORS1 data after April 1, 2007 and FORS2 data after April 1, 2002, you will always find 2 files per observations. You need the ones with DET.CHIP1.ID = CCID20-14-5-3 or Norma III.