The information gained played an important role in the German victory in the Battle of France.I’ve gone through the various statements on the solution of this code by German personnel here, here and here. Additional information on the FLD code is available from Dr Erich Huettenhain, chief cryptanalyst of OKW/Chi during that period.
The article ‘Erich Hüttenhain: Entzifferung 1939–1945’ by Friedrich L. Bauer in Informatik-Spektrum Volume 31 presents some case studies on cryptology from Huettenhain’s unpublished manuscript written in 1970.One of the cases described is the French FLD net:
‘Das französische fld-NetzDer nach dem Ersten Weltkrieg sehr erfolgreiche Dienst des franzosischen Kriegsministeriums benutzte im Verkehr mit seinen Wehrkreisen einen vierziffrigen Code, dessen 10.000 Gruppen fast alle belegt waren……’
According to Huettenhain the crypto system was solved by finding messages that had parallel passages. The system was identified as a transposition cipher.From mid 1939 the code was used by the French War Ministry in communications with the military district adjacent to Italy. However from September 1939 this method was adopted by the other military districts too!
Huettenhain says that this success allowed the Germans to monitor the situation in the rear areas, the movement of troops, the armament of the individual units, the morale of the troops, the weak points of the Maginot line etcUnfortunately this German success has not been recognized by historians.
Acknowledgements: I have to thank Frode Weierud for sharing the article in Informatik-Spektrum.