By the way, are there PC-9800 versions of the SCCs like there is for the LAPC-I?
The PC-98 equivalents are the GPPC-N and GPPC-NA. AFAIK, a SC-55mkII-compatible GPPC card was never released.
Also, the GPPC daughtercards are the exact same boards as found in the CM-300 and CM-500.