They'd mux the segments rather than the digits. That way, a given segment, in each digit was controlled by a single resistor, generally in the common-anode. Rather than multiplexing the digits, they'd concurrently drive the 10, more or less, digits and sign. This is easy to see if one is in a dark room and moves the calculator around in a circular motion, with all 8's displayed. Some calculators went further in that they displayed only half the digits at one time.

RE