My version of the little flash sync doohicky is called a Hakosyn. Comes in a little sheetmetal case that's got a 3C in the lower right corner and is also marked Made in Germany. I've had it for 20 years or more but I'd bet you can find one on ebay occassionally (not right now though, I just checked)
PS Picture being worth a thousand words, I just added one.
The gadget I referred to was a Leitz VACU (picture below), which fits over a specially modified version of the shutter speed dial. I am interested in this because I have a Leica IIIa with the special dial, which looks as if some lunatic has attacked it with a file. But no! It's meant to act as a trigger cam.
Another vote for the IIIf. I picked one up last summer (my first Leica) and it's such a fun camera to use. With the collapsable Summar 50/2 I put on it it's very compact and fits in a coat pocket in winter. In summer it hangs around my neck the whole day without being tiring.