As Willie says:
The wedge is to hold the barrel in place. It is not to adjust cylinder barrel gap. Adjust the arbor length and you can flatten the wedge driving it in and not change the cylinder/barrel gap.
This is far more common on the Ubertis than the Piettas.
Only problem is that Pietta does not make a Walker.
My first BP revolver was a Remington as was my second, third and fourth. It was only after I tried an 1860 for the first time did I sell all of my Remingtons.
I bought both Uberti and Pietta 1860s until I ran across the arbor problem. I now have a bunch of 1860s and they are all Piettas.
Current favorites are 5.5" barrel sweethearts. Polished up, cleaned up, new springs as appropriate and latest, Slix nipples (Hot Shot variation).