How to use an ASY2400 Xenon Flash Unit
You can almost work out the circuit from the photograph, since the PCB traces are fairly visible through the substrate.

The circuitry to the left of the power switch generates the high-voltage to charge the flash capacitor C3 at the top. The circuitry to the right of the power switch provides the trigger pulses to the trigger transformer T2.

There's a diode between J5 and J4 &mdash; it looks like the intent is that you connect an external power supply to J5 and J1, and then hook a battery between J4 and J2. This allows the external supply to charge the battery.

J3 and J6 are a little less clear. If I had to guess, I'd say that J3 is another ground connection. You can verify this by looking at the other side of the board. J6 looks like it's connected to the high-voltage bus, perhaps through a resistor, so perhaps it is meant for monitoring the voltage in some way.