TLDR: The holes at the entry of the keybox aren't wide enough, and can preventing high notes on the melody strings.

The holes where the strings enter into the 6.3 keybox doesn't allow for much lateral string movement, and if you press the keys too far (especially on the high notes) the string will contact the keybox and stop sounding. There is only 3 mm between the string and the keybox edge, so if you press the keys more than that or if the string vibrates more than that there will be problems.

To complicate the issue, if you use 1.5 mm thick veneer on the wheel (the instructions call for 1.0-1.5 mm

thickness), your wheel will end up on the order of 141 mm outside diameter. In my cases it was 141.4 mm OD. This large wheel OD will cause the melody strings to sit very high at the entry to the keybox. With the string sitting so high, when you play higher notes on the melody strings the strings can contact the radius on the upper left corner of the entry into the keybox, put there to reinforce the string lifter (or mute) that's built into the keybox entry.

Keeping the wheel OD down closer to 138 mm will help avoid the radius. You can do this by using thin (approx 0.7 mm) iron-on veneer on the outside of the wheel, after shaving the bare wheel to be perfectly true. This is readily available at most large hardware stores in the US. My wheel done that way ended up at an OD of 138.3 mm.

But the 3 mm clearance (before pressing any keys) still seems to be too small. So I made the entry holes wider, and now have 6.2 mm between the string and the edge of the keybox, and added a separate string lifter to replace the ones I cut away. To keep the cuts straight I made a 3d printed cutting guide

Print these face down on your build plate. The string lifter stl download is standing up to start with, make sure to lay it down flat as well.