If you're referring to the vacant bracket and two-pin round screw-in connector next to it, those are for flash sync. Good luck finding focal plane bulbs these days (even if you manage to locate a usable "light saber" Graflex-mount flash), but you could still use a modern flash unit with a curtain opening that exposed the entire frame for an instant, if the sync fires at first curtain full open (it probably fires at first curtain start, FP type bulbs would burn long enough for a Graflex or Speed curtain to make its full travel even on low spring tension settings).