Are we still absolutely postive that's what it is? I'm serious about not being 100% sold on that. Does *anyone* have another example of a '040 "upgrade" that went into the PDS slot of a Quadra? There were PPC accellerators that fit in the PDS slot, but I haven't seen any evidence that they made 68040 cards for it, and for good reason: any such card would be *barely* faster than the built-in CPU. (And heck, even if you did think it was worth the effort, I'd think a better way to design it would be as a clock-multiplying daughterboard that plugs into the CPU socket; every desktop '040 machine has a socketed CPU and that was standard fare for 486 upgrades.) It still seems to me this *could* be some sort of math coprocessor/DSP/"Photoshop Accellerator" card.
I don't want to suggest prying the heat sink off to see what that chip really is, but I'm still saying it would be a worthwhile exercise to count pins on the socket (it looks like it's through-hole so the pincount should be visible from the back) and verify that it's at least the same socket as a 68040.