I recall it is well in excess of 6A as the flash is first recovering from being discharged on manual. I was using a 5A maximum deflectiion analog ammeter with a bench power supply (old PC power supply , 20A, with the regulator fooled to put out 5.7V by sticking a diode across it's reference terminal), and the meter definitely was pegged for a few seconds.
The bench test confirmed the pack was viable, so I went and spent $30 on an about 4-5Ah 6V AGM battery to power the Metz pack, and still be able to fit it in the battery comparment. I have since adapted the pack to allow exteral 6V batteries to be plugged into the pack to augment the power reserves of the internal battery.
I had no charger for the pack, so I sniffed in my big of spare power bricks, and found one 6VAC 800mA for an old adding machine that had been discarded. I made up a pair of contacts to suit the pack, and then fed the pack with this thing plugged into an autotransformer. The voltage was wound up until the battery saw the charge voltage it wanted to. I think I am running the power brcik a feed woltage of about 140VAC, but it has worked fine this way for the last 4 years, with intermittent use.