In the meantime, take a look at this:
2.2 Appellant/representative reference
Appellants or representatives may indicate their own reference of no more than 20 characters in the space provided. e.g.: initials, string of letters, etc.
http://oami.europa.eu/pdf/forms/appeal_note_en.pdf
2.2 Referentie deposante/gemachtigde
Deposanten of gemachtigden kunnen hun eigen referentie van maximaal 20 tekens in de aangegeven ruimte vermelden: initialen, letterreeksen en dergelijke.
http://oami.europa.eu/pdf/forms/appeal_note_nl.pdf