I've seen some pretty huge generic protoboards with edge-rails for connectors like that before, but I also second the notion that this may well have been a case of someone just happening to have this thing handy and using it to prototype some random thing that has nothing to do with NeXT. The only thing about it that at all suggests to me that *maybe* the board was intended to be seated into a NeXT chassis while running is I didn't see in my cursory glance at it an obvious place where they would have hooked up an external power supply and they were apparently using the "5v" and "ground" rails as labeled... which means it *could* have depended on the 5v rail coming through the NuBus slot connector to power it. But there may well be a wire to, say, one of those 9 pin connectors that provided the power feed, or something else that was snipped off.
(This was probably a very expensive board back in the day, certainly by protoboard standards. It's looks like it's multi-layer; the only thing the multi-layer-edness seems to be used for outside of the unpopulated NuBus connector section is for the power/ground rails, but that alone is pretty fancy.)