Will a 20mm strap fit a 21mm lug Tag okay?
Why start a new thread for this question? It'll fit but it will leave a 0.5mm gap each side and not look great, so better to squeeze in a 22mm strap. Or better still buy a 21mm strap (they do exist) or have a custom strap made.
But the thing is 22mm sized straps does usually have 20mm buckle size and there is not much 21mm strap choice
Have a custom one made then - you can specify width at the buckle as well as the lugs, and also choose the material, stitching colour, etc.
Heuerville are very good but not the cheapest:
But there are plenty of other places that do custom straps for cheaper.
Separate names with a comma.