Thanks for the help @Bri7
The boots measure up to 20 [mm] whereas the bindings give either 12 or 16 [mm] if I measured it correctly...
I guess I'll bring the boot's along the next time I go binding-shopping, another valuable internet-shopping lesson

Recently I've bought my first pair of 3-pin bindings (shown in the image below...). They come as an unused reserve-pair from an avid pair of cross-country skiers who bought them 15+ years ago and never used them. Being a telemark skier interested in some longer cross-country tours in the next winter...