I have a few PC sync to 2.5mm cables like this:
Sure it can be plugged into the IIIF's flash port, but the port doesn't hold it tightly. It's very loose. On the other hand, it fits nicely on the Nikon S2 flash port.
Now I wonder, perhaps, I have to use the original Leitz cable, like this?
I have a few PC sync to 2.5mm cables like this: View attachment 398316
Sure it can be plugged into the IIIF's flash port, but the port doesn't hold it tightly. It's very loose. On the other hand, it fits nicely on the Nikon S2 flash port.
Now I wonder, perhaps, I have to use the original Leitz cable, like this? View attachment 398317
What you really want is a cable with the long pointed connector in your first photo which plugs into the flash and the little Leica connector in your second photo which attaches to the camera. (Apologies for the technical terminology ) I have a similar issue using my Metz 35 C-2 flash with my IIIf and IIIg. I have never found such a cable.
But I did find the cable on the left below with the Leica connector and a female PC connector and the cable on the right below with a male PC connector and the long pointed connector.
And here they are connected together.
It isn't a very elegant looking arrangement but it works.
I bought an old leica flash cable on eBay and took the connector end from it to construct my own cable - it's only two conductors and was pretty easy to assemble. The locking ones meant for the barnack bodies attach nice and securely.