I assume it is a socket for an alternative synchro cord (with miniphone plug).
In case it is you should be able to measure (by means of appropriate plug) the same voltage as at the pc plug (you would better use an approprite adapter with easy to access terminals).
that makes perfect sense ...
i have to admit i have never seen a miniphone plug sync cord before .. thats what threw me off
and since it has a regular pc adapter plug, im not going to even bother using it ( the mini plug )