Why are there holes in Norwegian coins

Information about Danish coins

The reason for the perforation lies in the collapse of the Scandinavian coin union.
In 1873, a common currency was created between Denmark and Sweden, the krone at 100 ore / Øre at 0.405 g of gold. The convention stipulated that the coins of all participating states were valid in the other states. Norway joined in 1874, so that from 1873/74 the three kingdoms minted coins whose mass, weight and fineness were identical. In principle, then, similar to today's euro coins.
When the 1st World War broke out in 1914, the obligation to redeem banknotes for gold was suspended. The economies of the three countries now developed differently, so that the crown rate began to diverge.
Regardless of this, a krona in coins was a krona, regardless of whether you were in Norway, Sweden or Denmark, so that speculations arose that, for example, 100 kroner in Norwegian coins was exchanged in Sweden for a Swedish one hundred kroner note, back in Norway these Swedish hundreds then exchanged for Norwegian banknotes and thus gradually achieved a not inconsiderable profit. The result was export bans for small change.
To put an end to this speculation, an additional convention from 1924 made it possible to issue "special dividing coins" that were only valid in the respective issuing country. The Scandinavian Coin Union was practically dead, but formally it existed until the early 1970s.

This is where your coin comes into play. The three SMU members did the following in 1924:
Norway struck the values ​​of 1,2,5 Øre unchanged, 10, 25, 50 Øre, 1 crown in Cu - Ni with the same format with central hole

Denmark . 1,2,5, 10, 25 Øre with central hole 1, 2 crowns in Cu - Al, a value of 1/2 crown was newly created. From 10 Øre significant changes to the format

Sweden: no change in format or alloy until 1942. Only then is the fineness of the coins reduced to 10, 25, 50 ore 1 and 2 kronen.

The perforation came and went every now and then. The Norwegian coins were not punched between 1951 and 1997, but the new coin series introduced from 1994 required punching of the values ​​of 1 and 5 Kr, the Danish 1,2 and 5 Øre coins were only punched until World War 2, 10 and 25 Øre not in the 50s and the values ​​of 1.2.5 crowns only from 1992.

The last survivor of the SMU today is the Swedish one-crown piece, which still has the diameter determined in 1873, in 2013 it will be replaced by a smaller-sized coin, from this point on nothing will be done except for the identical name of the three currency units to create a common gold currency in northern Europe almost 150 years ago.