You have the insert for 4 x AA batteries. You need to put them so that they alternate in polarity. They are wired in series to give a 6V charge. The 22.5V version is like this one:
https://www.ebay.com/itm/Ernst-Leit...580134?hash=item23a35c2426:g:cWAAAOSw9GhYgViG
I prefer the 22.5V version. It is a battery capacitor unit, so the bulb firing tends to be more reliable. The capacitor is usually dead, so you have to take it apart to replace the capacitor. 150uF to 250uF 25V electrolytic capacitor will do. I also put an A23 battery (12V) holder into the location of the 22.5V battery holder. There are a couple of reasons why.
First is cost. A23 batteries cost about $2 each at Walmart. The 22.5V batteries cost over $10 each. Furthermore, the currently available replacements for the 504 are too big, so that you can't put the cartridge into the flash body with the cell in it.
Second, the A23 being alkaline have a better discharge curve than the dry cells, which tend to die all of a sudden.
The upper terminal on the holder is positive. You will notice that the positive terminal seems to just stick up into the air, connected to nothing. This is the Leitz secret sauce!
When you put the battery cartridge into the flash, the battery lead touches the undersurface of the flash head itself. The circuit to the rest of the capacitor is complete only when you put a bulb into the circuit. It will charge through the bulb itself. This feature helps prevent you from burning your fingers inserting a bulb if the PC cord is somehow shorted.
