Can't speak for Agfa MCC but I can say that Simon Galley of Ilford has said that there is no incorporated developer in Ilford FB so whatever you were washing out, it wasn't that. You'd think that if anything needed washing out, Ilford would mention it in its very comprehensive instructions and it doesn't as far as I can recall.
No clue what it is. Could be an accelerater??? All I know it worked. May have been MCC. Although it still wasn't that nice after Lithing.