This is super cool - I haven’t ever seen these before. Seems like a potentially interesting way to add external video to the SE/30 using readily available NuBus video cards.
Yes it would have given the SE/30 access to all of the cards available for Nubus.
Even for an SE model the idea of 4 SE PDS slots is mind blowing providing the possibilty for acceleration, a 2 page monochrome display card, 24 bit colour display card, a DOS emulator card or a network card!
7.2.1.4 PAL DEVICE. A 16L8 PAL is used to generate chip select signals to the DDLC and '374 address expansion register. The base address of the DDLC is 520000h and the address of the address expansion register is 528001 h. 520000h was chosen as the base address because it is in an area that does not have RAM and is not used for Apple factory testing (DC&D 13-16). The PAL also generates an inverted MCLK for the DDLC. When the card is installed in the Second Wave ExpanSE chassis (available from Second Wave, Austin, Texas, (512) 343-9661) the inverted clock is not used and MCLK is connected directly to the C8M signal of the chassis. From: 1989 MC145488 Dual Data Link Controller
The chasis by Second Wave that allowed you to connect your old Nubus cards to a newer PCI Mac was called Xpanse PN series and was available from June 1995.
Pricing at the time was $595 - $1895 for a 2-, 4-, or 8-slot NuBus expansion chassis.
I guess this was a way to market the same thing that had been around now for about 6 years as something new with the aid of the PCI adapter.