I am pretty much certain that it's for an SE, having now taken one of mine apart for the first time. The slot is the right size, and the board would fit across the logic board and inside the case nicely.

And there is indeed a similar — but not identical — SE Ethernet board ("Technology Works") over on Applefritter's Nubus mafia pages.

SE/30 PDS slots are longer than those of the SE. Yup. I took the trouble to compare the two in the last 12 hours, after UnknownK's post.

Not sure that I want to fry either the SE or the board, however, so I think I will wait for 99.99% confirmation before firing it up.

Now, what to do with it? An SE webserver is certainly a temptation....