So, the following little module is able to store a 16 bit value...and will be used as the ALU result register. Two little boards will be next, one as input enable into this register, one as output enable from this register to the data bus.

You see 17 relays for 16 bit...thats because one relay is used to switch out the 24V to clear the register, this is done by a "Clear" input signal.