Does anyone know who produced this slot expander?: Yahoo Japan Auctions link. The current bid is 20,000 yens

ESE Artists' maybe? (the seller is selling at auction other ESE's MSX stuff).

Where found msx slot component?

Try 72000!!! (WTF?!?!?)

72,000 yens = 446 €

By the way, this slot expander is not from the ESE Artists' Factory, but from Frontline.
Frontline was a Japanese MSX circle active in the late nineties led by Yokoi Hidekatsu. This circle was the basis of what later -with a few members added- became the MSX Association.