The screw cover on the cable release socket may have been introduced in the later models of the 518/16. I think on models with the anti-double X assembly. In these models, you will notice a different shutter button as the cable release socket is now incorporated like modern shutter buttons.
I have the 518/16 Novar f:4.5 with Pronto shutter, this has the later anti-double X. Based on info found on the net, circa 1955-56.
Actually, the 518/16 has two cable release sockets - one on the body release and one on the shutter housing. I was not aware of the
one on the body release - I just never looked that closely, I guess. The shutter housing cable release socket can be clearly seen in this picture from Marriott's World.
Best idea would be to look for an inexpensive non working camera on ebay for donor parts. I have this camera with the f6.3 lens. Makes great negs/images!
In my case, when I got the 518/16, I did not notice the cable release socket in the shutter button. I was preparing to check for the accuracy of the focus and tried to take out the cover screw on the one in the shutter assembly, could not find a screwdriver to fit, then I noticed the one in the shutter release button so problem solved.
Yes, it will be simpler to find a donor unit, but the quality of these cameras are good. Unless they were exposed to the elements, they can be revived...
Yup, I found the lever and I'm able to fire the shutter now. Thank you very much. Now I just have to find a correct size screw, unless it's a special design screw