Having been involved in the development of some of the UK road signage during the late 80’s and early 90’s I am not aware of an earlier official sign for low flying aircraft. Variations that have been used include the warning triangle with a blank centre and also the exclamation mark, both of which have a supplementary plate with the words, “aircraft noise”, ” sudden aircraft noise” and “Low flying aircraft”. There may have been other ‘local’ variations including the warning triangle with a plate saying “caution jet blast”.

Contrary to the belief of some motorists, the wind sock on a warning triangle indicates sudden wind gusts and not the presence of an airfield.

Unfortunately Irecently threw away my design notes for road signs, but the aeroplane symbol as shown on the warning signs and also direction signs to airports is set prescriptively set out on a grid on Department for Transport WM/WBM series drawings. The aircraft shape is very generic and given the sort of people involved in highway design specification, it is unlikely to be based upon a specific aircraft.