I thought one could compare several advertisements from the same company, to see if they fixed any typos. The first one we've looked closed at was from February 1981.
This one is from November 1980. While the MB-6890 did exist at that time, the reseller doesn't seem to have specialized in it yet.
The next issue online is not until September 1981, by which some of the items seem to have expired. However a slew of Commodore stuff was added in the mean time.
At least we can compare some of the prices. Many seem have dropped by 33% between February and September 1981. If I get Google Translate correctly, the items on the bottom right page were a compatible case for 35,000 yen, an original keyboard for 25,000 yen and an original power supply also for 25,000 yen. Exactly what kind of original those were, the image of an Apple ][ like computer as well as the text box with Applesoft Tutorial and other items should be give aways. That makes me wonder if the items we already looked at belong to the Hitachi MB-6890 at all, or rather were part of a DIY Apple ][ clone with optional Z80 and 6809 CPUs.
Perhaps in February, they only had generic cases (16,000 yen) and that you would buy a cable to attach a standard keyboard to your Apple ][ clone, but in September they had good enough lookalike cases and keyboards to supply instead.