Is the image above a render, I’m assuming? Looks great

Dude this would be sick. Definitely not an abomination in my opinion. Like you said, with the movement being the Sellita sw-200 base it should be doable if the dial diameters are the same between the two watches. You would also need the movement holder from the 556.

From what I’ve been able to find on a web search, the 104 has a dial diameter of 32mm and the 656 has a dial diameter of 30mm.

I couldn’t find the dial diameter for the 556 listed online, but I have swapped my 656 dial/movement/movement holder directly into my 556 case without issue so it’s safe to assume the 556 dial diameter is also 30mm.

Bummer, this would be a rad Isetan 2.0 version 556 if it were possible.