Additional Input To Old Post: I bought a used 45-CL1 with a battery holder for Alkaline batteries (based on position of the negative post). The CL version is the newest of this model. It is my understanding that Metz never made a battery holder for the end user to add their own rechargeable batteries. The only rechargeable battery holders are the Metz made ones with permanent batteries inside - either NiCad version or NiMH.
My battery holder DOES have a capacitor wired in. I assume it is a capacitor anyway. My guess as to the purpose is that it is an improvement to the older battery holder for the Alkaline batteries. The main problem with the Alkaline batteries is the voltage and battery capacity decline rather rapidly. The capacitor may assist the batteries as they start to decline, almost like an extra battery. When the main flash capacitor is charging, the small capacitor and the batteries are discharging. When the main capacitor is done charging, then the small capacitor can recharge from the batteries getting ready to assist during the next cycle. I am not an electrical engineer or anything close to this, so I could be completely wrong.