Thank you all for your replies.
I believe capacitor in series with the resistors are used since they resist change in voltage and this may limit transient voltage to some limit.
To protect the opto you just insert a Zener in parallel to the opto-LED. There are special surge supressor diodes for this usage. Your dealer should be able to help to find the right one. For a quick'n'dirty solution you could also use 3 or 4 normal diodes in series, and this (in forward direction) in parallel to the opto input.
I need to either use the zener diode or the surge suppressor diode. Not both right?
One more question. I'll need to make 16 such identical units for the demonstration of my project. What is the optimal solution for my problem, not too expensive, yet protective? (Combining elements of the above suggestions)