I have seen some discussion on AC websites about this colour which was apparently used on the Ace and early Cobra's as well as on some Bristols, one paint shop had posted pictures and said the closest they could get to it was Guardsman Blue, the metaline referred to the type of paint not the colour and on a former posting on this site there was a photograph of a Bristol colour chart with several metaline blues on it but none with the reference number M651.
Not sure if the link will work if not try searching AC Ace Colours metaline blue or Guardsman Blue.
AC Ace Colour - 1953 AC Ace 2.0L(AC)
Geoff.