90-91 sedan and hatch flaps are interchangeable. The curve is slightly different but they are the correct length and all the holes line up. The 88-89 is not like this, unfortunately. You can use 88-89 sedan flaps on a hatch but they don't fit very well.
Here's a comparison, 91 Si and 91 DX sedan. Both cars have 90-91
sedan rear flaps. (FYI all the front flaps from hatch and sedan are exactly the same 88-91)
This is what zenki flaps look like on a kouki...a little too short and the holes don't line up but it fits nicely.
