Switching two optocouplers based on different voltages

I don't think it's productive to ask "what are these for?". The reason I don't think it's productive is that frankly I don't think the designer had any idea what he was doing. Putting an optocoupler in the B module - well, sort of. Trying for noise immunity, I guess, and maybe level shifting, as has been mentioned. Driving it with an optocoupler in the A module is just silly.

I suppose the lightning hypothesis is the best bet, but with both common ground and common power acting to lead any such surges back into module A, it's wasted effort.

Something like this:

^{simulate this circuit – Schematic created using CircuitLab}

EDIT: This circuit is a wide input control voltage. But what you have asked is an ESD protection for optocoupler. IMO you don't need it, the optos are tough devices.

EDIT2: I am in PLC business for more than 15yrs. I had never seen a PLC with opto isolated input having TVS protection. The only failure I have seen was due wrong connection: 230VAC instead of 24VDC. 5m meter long wires are very short in automation world. A more probable failure that has to be expected is due to some contactor/relay kickback voltage, that is not equipped with freewheeling diode or MOV. In such case a negative spike will travel along your power supply. The reverse diode on the optocoupler is a must, or alternatively you can install an opto with bidirectional LED. It can provide you additional feature, if you want to use positive (PNP) or negative (NPN) logic.

EDIT3:

Your schematics is too complex. too many components. Here is a simpler one:

^{simulate this circuit}

High speed with addition bleeding resistor, If LED = approx. 10mA:

^{simulate this circuit}