Nettar models are listed here...
http://camera-wiki.org/wiki/Nettar
All of the ones that end "/16" are 6x6. The 515s have a flip-up finder, 516, 517 and 518 all have a built-in finder. The 518s had the most options for shutters and lenses. The main issues I've found with them are the black paint flaking off, the shutter speed and aperture controls getting a bit stiff, and slow speeds getting sluggish. Sometimes the focus gets a bit stiff. The B setting has been fine on the ones I've seen. The stitching on the case around the bottom edge can be shot (although tends to stay attached at the bottom front edge).
The signal Nettars are so called because they have double exposure prevention - can't cock and release the shutter twice without advancing the film. Some have a little red flag in the viewfinder, and some have a hole in the top cover. Beware that it's possible to trigger the mechanism without actually releasing the shutter - with a slow partial press of the shutter button, you can hear a faint click. If you take your finger off the button and then try to press it down, it won't release the shutter.
At first glance, the screw in the cable release socket doesn't appear to do anything, but it's a workaround for the shutter mechanism on the front not being connected to the interlock for double exposure prevention. There is also a cable release thread in the shutter button, and the clue is in what happens when you use a cable release in each. With it screwed into the shutter button, you get double exposure prevention, with it screwed into the shutter mechanism at the front, you don't. Since the lenses and shutters were also fitted to models that don't have double exposure prevention, the easiest and cheapest way to encourage people to use the 'safe' cable release socket on the shutter button was to stick a screw into the socket at the front. So, if you tend to use the shutter button and trigger the double exposure prevention thing without releasing the shutter, you can always get past it by sticking the cable release in the front. To save losing the screw, just take it out right away and stash it in some container for storing small parts (empty 35mm film cannister - labelled!). It goes without saying that, with any folder, it is definitely good practice to always advance the film immediately after making an exposure.
One other thing to keep in mind - it has no strap lugs. So if you want to carry it around on a strap, it needs to be in the case (and the case does not separate into two parts). The captive screw in the bottom of the case has a tripod socket, so you don't need to take it out of the case to mount it on a tripod. If you're only using it with a tripod, just dispense with the case and keep the camera in a bag or jacket pocket.
Regarding fitting the pinhole, the easiest and quickest is to just take the whole lens/shutter assembly off and tape the pinhole onto the front. That will give you something usable right away while you look into the options for getting the glass out of the housing so that you can refit the shutter for handy B exposures with a cable release. As I said, getting some of the elements out could be tricky (and would be best done with the assembly taken off anyway).
They're nice wee cameras to use with a lens as well - light and compact, and the square format means it's always the right way up. If you end up looking for another to use with the lens, seek out one with plenty of shutter speeds. Some have a very limited set of speeds, while others have lots of slower speeds. f4.5 is a good max aperture to go for - the f6.3 is a bit limiting. There was also an f3.5, but they're pretty rare.