One bit refers to it having exactly one bit of what could be called "architecture state" or "datapath", that is the second channel of lower 74C374 going from in

One bit refers to it having exactly one bit of what could be called "architecture state" or "datapath", that is the second channel of lower 74C374 going from input to A0 of program store. "Instruction" has two phases and result of first phase determines what will happen next by means of this one bit (by changing which pointer to next instruction will be read from memory).