One of the links refers to a post I made. I was wrong - the solder pads were for a 64 pin ROM SIMM, not a 72 pin SIMM. I didn't really look at it much before reading that some Quadra 630-type motherboards have two SIMM sockets, but realized when I pulled out the motherboard to add a SIMM socket.

Needless to say, 128 megs (or 132 including the built-in memory) isn't bad for a Quadra 630-type machine.