it will most likely turn into a 21mm strap through shrinkage. Most of the leather straps I have purchased shrink a mm or so. Compressing leather like that 1mm is not a lot.
I have a Traversetolo Vitre myself and fitted a Jurgen high polish mesh bracelet to it. I started with a 22mm bracelet and carefully ground down the endpieces until I got a nice fit and then buffed it back out. It came out great.