Th note was written for "1,000 Carolus Guilders of 20 Stuivers a piece." I believe the Carolus Guilder was a copy of the Golden Florin, which was 3.5 grams of gold. A gram of gold is about $50 today, so that would make the value of the note $175,000.
At the reduced interest rate of 2.5%, the coupon should be $4375 per year. Instead they got $153 for 12 years of interest, a devaluation of 343x.
Its worth going through the math: 2.5% x 1,000 Guilders = 25 Guilders. The Netherlands replaced the Guilder with the Euro in 2002, with an exchange rate of 2.20371 Guilders per Euro. If we interpret the contact as being due in Euros, then the interest should be €11.34/year, or at today's exchange rate, $13.44/year.
If they'd reinvested their 2.5% every year, in a similar bond paying 2.5%, then they'd now be collecting dividends of something like $600,000/year... absolutely crushing holding Gold.
The fact that this is a perpetual bond is interesting, but rolling an investment through shorter-dated bonds paying 2.5% over the centuries would have achieved a better result.
Put another way, the loan will continue to pay out 14 Euro per year 100 years from now, when that will likely be the cost of one can of soda.
I haven't been able to buy government bonds/gilts with an coupon rate hire than inflation for years.
At least my equities return when everyone deflates the value of money.