Pretty sure that is to capture and route any "outgassing" that occurs at each stage.
Jeff nails it, those are venting holes, for venting dangerous gas in case diaphragm leaks or punched, and normally at the end of venting line is the reaction container filled with neutralizing agent.
So, the purpose of this setup is to evaluate flow thru the flowmeter using either the metering valve on the output or the input?
I was lucky to find the Kip 316 stainless steel double solenoid manifold. clippard, numatics and bestwick also make multiple ports manifolds, but use aluminum and the outlet ports are 10-32.
I think Kip inc. is own by SMC now, I don't know if SMC keep the stainless steel, 1/8 npt ported manifold in production, this manifold can not be found on SMC official website.
You ask the right question, the outer most solenoid is the main to initialize the flow to the flow meter, and the flow rate is decided by the valve(outlet) on the flowmeter, this flow rate can be accurately monitored because the pressure in glass tube is known.
when the outer solenoid is off, the second/inner solenoid allow the flow past the swagelok s metering valve, flow rate can be monitored through flow meter but it is an "estimated" value, because the pressure in glass tube is not known, how much the flow rate to be decided depends on observation.
The purpose of such setup is to inject the co2 in short period of time to rise co2 concentration by 20ppm, through outer most solenoid and the valve on flowmeter, this is relative larger flow and can be done right before the light turns on. The second solenoid turn on while first solenoid is off, the co2 flow rate is small and by how much is decided by observation, this flow of co2 keeps on with the lights on, and replenish the decreasing co2 in the water so the concentration stay between 20-30 ppm.