Slang & colloquial terms like this are just as much a kind of folklore as the songs we write of; and so will exist in as many variations. No doubt on subs in which Cyril served it meant a fried egg on toast. In others it will have meant a poached egg, or dried egg, or fried bread, or any combo of these. No more point our arguing about here about what it 'meant' than in our trying to establish a 'correct' version of Barbara [or Barbra or Barbary or Barbry] Allen.

~Michael~