She brought these two 10-guilder banknotes and joked, "Can I upgrade my ConFrancisco membership with these?"
(ConFrancisco, the 1993 Worldcon, was selected at ConFiction, the 1990 Worldcon in The Hague, Netherlands. This was pre-Euro and the site selection voting fee was 47 guilders; it was a foolish choice of price and really should have been set to 50 guilders, but never mind about that. After we won, we also accepted guilders for upgrades, but only at the convention. At that time, one guilder was worth approximately US$0.25.)
Based on my searching online, these appear to be World War II-vintage banknotes issued in 1943 during the German occupation of the Netherlands. They seem to have some value, as I find them advertised online at prices starting around $10. What we'll do with them, we haven't yet decided.