That's because they all use the same gun. The Willemoes and the Niels Juel class used the older 75mm Maskinkanon M/71 LvSa which is the Danish designation for the Compact.
It all gets rather confusing once you realize that the M/71 seems to be two different guns.
This is how it looked on the Willemoes class
This is how it looked on the Niels Juel class before their mid-life update in 1998
During their MLU the exhaust or whatever it is was plated over.
I'm guessing that had to do with the update the guns got.
And just to confuse the issue even more it looks exactly like the M/85, yet it's an entirely different gun underneath the shield.
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