The Sonnet Quaddoubler was an overclocked 50mhz upgrade that stuck into the CPU socket, so if you're going to do something stupid like this you don't *need* to use the PDS slot...
Actually, interesting point here: I don't see a crystal on that board, and the board was in a Q650. The PDS slot in that machine is going to be running at 33mhz. The QuadDoubler doesn't need a crystal because it derives its clock from the system clock simply enough by being phased 2:1 with it (which is why it only works in 25mhz '040s), if this runs at 50mhz it's going to have a much rougher time deriving the clock from the 33mhz signal it's getting from the motherboard. And... on top of that, the performance is going to be *lousy* because there would be a non-integer relationship between the card CPU's bus cycles and the memory bus, meaning you're going to be getting a buttload of wait states. (Unless the card magically switches to 33mhz when doing external bus cycles and only runs at 50mhz when it's not doing anything external... which I think is going to be "never" on an '040 because of the onboard cache doing line fills all the time?)
Again, totally not going to rule out it being an '040 acellerator, but it's a very weird one if that's what it is.