If that socket has the right pin count for a Math CoPro, my guess would be IIsi.

It's really strange to see those PDS contacts unshielded/unkeyed, which makes think "prototype" or "hack" right off the bat. The positioning of that (partial?) passthru connection makes me think IIsi as well.

Physical fit of the board would be the telling point, so far as I can figure.

How wide/long is the card?

Is there a matching hole hidden behind the connector in the shot for the one on the outrigger?

What is the height of those pins for the PDS.

More pics?