Reclaimed LEDs from fixture. What kind of power supply can I safely use?
If there was a series resistor, see if you can probe that for a resistance value. Then you can figure a forward current and voltage of each LED. If there was no resistor, I'd say your answer of 7.2V is accurate enough. If they were all in series, you know at least they all have the same forward current of less than 470mA

Each bit of information will give you a better answer, but nothing beats a datasheet. Any model numbers or identifying marks would be the most valuable bit of info