During their production run, Leitz changed the bulb holder anyway. It may be missing because it had the earlier mount.
Find the specs for the lamp, locate a current equivalent and hit the local electrical supplier for a socket. I don't remember if there was an adapter needed, it's been a Looong time.
The connections for the socket were crimped connectors. auto parts store should have 'em. Hell, if ya do it that way I'll(PM me) send you a couple of different sizes but its pretty thin wire with a high temp insulation.