The old sign for guilders was the abbreviation for "florijn" = fl
And the Dutch was = H therefore "Hfl."

This is sometimes still used, though the international exchange notations almost always are listed as NLG / Guilders / or in German "Gulden"

On the site: http://www.oanda.com/convert/cheatsheet?lang=de
you can look up the "Holländische Gulden" which is there abbreviated as NLG.

Your choice

Explanation see answers above.
This is what "Handelsblatt" is using. I'd definitely go for this.

NLG=Niederländische Gulden(holländisch=nederlandse gulden)
früher war die Abkürzung:HFL=Hollandse Florijn, Holländischer Florin)