What is this component? Marked R5
I think bidirectional or unidirectional TVS diode, but not sure. Because there are many in series next to what looks like a bus. Since the board is so old it is unlikely that you will find the SMT markings useful. It's a diode for sure. I'm guessing the 24 on the device stands for 24V, but there isn't away to know without testing. 

Here is what to do:
Unsolder a device from the board, get a benchtop supply with a constant current mode and slowly turn up the current (the steps could go something like this: 1mA, 5mA, 10mA, 15mA... ect all the way to 100mA) and not the voltage at each step. Once you get past ~50mA, you need to be careful, don't exceed 150mW total. If you need to go past 50mA to find the turn on point of the diode, do it slowly and only turn the power supply on briefly. Another thing to do would be to put a series resistor and measure the voltag only across the diode with a DMM

Plot out the voltage and currents, reverse the device and do it again. If it really is a TVS or zener, then it should be relatively simple to find a similar device after looking at the plot.