I have the 8531-II and have some parts from a III in it. I can't keep the part numbers straight. I found this info:
"Note there are three versions of gas valves that are applicable to the models listed below. Prior to ordering, please compare the picture of the valve to your existing valve to ensure that you are ordering the correct gas valve. If you have a 31096 or 31097 gas valve, you must order the 31155 and transfer the fitting, coil harness and the gas manifold from the old unit."
from:
https://pantherrvproducts.com/31098/I have a photo of either the II or III's with some other parts-burnt out burners etc. That shows how the gas valve is in a II or III (same). Not sure what is meant by "side" in the OP