If you get the types mixed the chip will get red hot and DIE!!
There's a few board versions each with a different chip, this goes for system 18,32 and Mega CD.
Some will swap around but it's to do with the time it was manufactured, not so much the hardware type.
The board layouts are changed as the chip changed over the years.
> Go from the silkscreen under the IC, make sure it matches your donor board EXACTLY.