I've just had a look on my wife's old yellow Petzl helmet, and there appear to be a load of numbers and letters on the semi-circular (roughly) bit of plastic that is part of the chin strap (the bit that sits near your ears), and which says 'adjust' on it. You have to pull the strap through to see them. The numbers on hers are: AO135OV (I think) on one side and AO125OM (I think) on the other; does that help?