From what I know, there are two variations of PC-8800 series keyboards regarding the legends.
The earlier variants feature katakana legends, have no ~`{} keys and have , instead of 、with the ネ(ne) key.
The later variants, the ones with split spacebar, feature hiragana legends, do have {} keys, have ¦(earlier variants have |), have ~ where ` is in modern keyboards but still have no ` key.
It is somewhere in the PC-9800 series era that the ` key was introduced to the NEC keyboards, although it was initially placed where the ~ is in modern keyboards.