Re: flip top. No, it's not a flip top but the top was made in two pieces - 1) the threaded area and 2) the cap itself. And the two pieces were soldered (or brazed) together. The gloppy things shown inside the top are the clumps of solder. The top is damaged, the threaded area is bent and the cap is dented. I'm not sure that the cap part was originally attached to the threaded area as it is not what I'd describe as a high-quality solder/braze job. In fact you can see daylight through some of the area where the two pieces are attached together. I kinda wonder if originally something else was affixed to the threaded area and removed being subsequently replaced with the current cap - but this is just blind speculation.
Re: ink pellets. I soaked one of the pellets in water and it does not seem to dissolve. The pellets are pretty hard, as in lead shot. And after dumping them out on a paper towel, the towel was covered in what looked like pencil lead. Also, a friend who is a retired Navy EOD, suggested testing the flammability of the pellets and the pellets are not flammable.
The mystery continues...