FNB = Danish contract with Fabrique Nationale, Belgium?
Can you provide some information on projectile types, loading date and headstamps?
To my knowledge the Danish navy phased out the last 20 mm Madsen guns in 1965.
The ammo of the FNB contract were ordered by a southeast-asian state. I don’t know the exact country (someone else here do?), but there a pictures of Thai Commando V150 armoured cars armed with 20 mm Madsens. Maybe a hint.
I do not remember in detail (and might be wrong) but there was also some talk about the Malaysian Navy.