i have a working gt 18 but i don't have the manual
i was wondering if someone familiar with this specific flash can answer a few questions about it for me ?

there is a plug on the back near the 'ready light"
it looks like an old school 1/8 headphone jack plug ... can you tell me what that is for ?
i have the wall plug, seems fine and it fires with batteries too, its just that socket ...
is it to daisy chain other flashes to it ?