Hey spijkerfestijn,
601 Sqdn was equipped with the Airacobra Mk I from late-1941 to spring 1942, operating out of Duxford. USAAF 350th Fighter Group formed at Duxford during the same period, equipped with P-400 and P-39L models. The range is only about 150-200 miles(?) from Duxford to Goes, Zeeland, but I have no idea if they flew operations over Zeeland. 601 Squadron did fly at least 1 mission over Dunkirk on 9 October 1941. Could the drop tank have drifted from the channel area near Dunkirk to the position where you found it?