A message had been seen on 18 June about courses lasting from the 20th–26th at Rechlin and Messerschmitt Augsburg, “to instruct personnel in the new employment of Me 109 and Me 110 with bombing equipment”. By midnight on 23 June it was learned that Köln-Ostheim was where a “Gruppe 210” would be re-equipped but within three days and “for simplicity’s sake” KG 76’s base servicing company was instead to be brought in from Lippstadt rather than using III./St.G 77’s. This led Luftflotte 3 to query exactly who was where and to stress that the 1 July deadline for establishing Erprobungsgruppe 210 (named in full for the first time) must be met. The 29th brought the news that “3 Staffeln of Stuka 77 and Flughafenbetriebskompanie [= Aerodrome Servicing Company] II./KG 76 are meeting in Köln-Ostheim for Gruppe 210”.

No more was heard until 11 July when the Aerodrome Regional Command at St. Omer urgently requested three bomb hoists; 40 life jackets; and 20 seat- and 20 back-parachutes “for sea flights”. The same report disclosed that the unit was equipped with the Bf 110 and was at an “HQ called ‘(L) 5/VI’”. In fact Erpr.Gr. 210 had arrived in Denain from Ostheim the previous evening and the HQ referred to was Fliegerhorst Kommandantur [= Aerodrome Command] E 5/VI which was at St. Omer.

Although apparently not deciphered in time to give a warning, a message of 13 July set out Fliegerkorps II’s intention to attack shipping in an area bounded by Dunkirk, a point about 43 km east of Foulness, Southend, the Somme Estuary and Beachy Head. Detailed to take part were IV./KG 1, II./St. G 1, “ZGr. 210” and KG 2. This is now known to have been Erpr.Gr. 210’s first action, against two convoys in the Thames Estuary.

Deciphered messages meant the British knew of this unit before its formal establishment; they also learned of its deployment to an operational base and of the orders for its first action.