1. Just spotted that the 2 'Fn' keys are unnecessarily repeated: in the base-kit and in the Add-on set.
2. Perhaps the two diamond keys on the Add-on set can be 1.5u to resemble better the meta keys of the HHKB.
Otherwise, one has to use the 1.5u Alt keys or 1.5u Ctrl keys for the HHKB layout.
ANSI (for comparison)
ISO UK
ISO German
ISO Swedish
ISO Greek
For reference:UK international kit for Round 6 (by 7bit)
DE/DK/NO/SE international kit for Round 6 (by 7bit)
HHKB support
A previous run of SkiDATA+
It actually has 65%/75% support and 1800 support. But it only has 1 HHKB control key, lacking the stepped HHKB control key option, and doesn't have the 1.5 meta keys for the HHKB layout either.
On the other hand, its ISO support was perhaps quite well examined and it has other goodies such as 1u backspace and a few LED key caps etc.