If I understood correctly, the characters corresponding to the first row of CP437 are actually 2-byte sequences (01 xx) on MSX, and the prefixed values as indiv

If I understood correctly, the characters corresponding to the first row of CP437 are actually 2-byte sequences (01 xx) on MSX, and the prefixed values as individual bytes are shifted down a row to correspond to the uppercase alphabet rather than being values 0-31, so iconv would have to know about the prefix to not mix those up. The resemblance to CP437 probably isn't a coincidence, though. Microsoft was heavily involved in MSX, so it was undoubtedly indirectly influenced by IBM PC.