Are you sure that your SE is an early 800k model? The 20SC disk was available on 1.44Mb models, too.

A 1.44Mb disk can be formatted as 800k by taping over the square hole on the right side of the disk when viewed from the top. However, this doesn't always work, and if it does, it usually results in a decreased disk lifespan. But all you'd be out is a floppy disk, so it's not that big of a deal to experiment.

I'm not sure what that card is. I'd say a specialty serial card of some sort. But yeah, crack the Mac and try to find identifying features on the card itself.