Here's my conclusion on Greg's bottle that started this thread. There is little doubt it is a mid-1970s Jam Factory piece, made by one or more of the trainees (Peter Goss, Rob Knottenbelt, John Walsh and Tom Persson) and/or Stan Melis who was overseeing production from 1976.
Several characteristics point to Rob Knottenbelt as the maker, or at least the dominant member of a team. Knottenbelt was the first of the trainees to start at the Jam Factory and so was more advanced than the others in those early years. His works of the period show a closer following of the design ideas of Sam Herman than the others, so the obvious SH attributes in Greg's piece also point to RK in particular.
The random swirling trails marvered in, and the squared lower form with its deeply indented base, both appear more characteristic of RK than the others at the Jam Factory. The 23cm tall green vessel in the photos below shows both of these traits. It is inscribed "R J Knottenbelt 1977 R2? SJH Studio" and dates from the year when the trainees worked toward a graduating class exhibition. I will also show a bowl with silvery brown trails and a vaseline lip, which has these same features.
Even more strongly identifying is the internal effect that at a distance looks like coloured flocking, but close up shows the colour collecting around tiny random bubbles. That is an obvious feature of the green vessel in the photos, and shows even more boldly in the bowl with brown trails. The same feature is found in RK's production work for years after he left the Jam Factory, although it does not occur (as far as I am aware) in the works of any of the others. I can see this same feature in Greg's bottle, perhaps not so well developed as in the two examples I show, but that would be explained by it being an earlier piece.
I tried to contact RK to confirm my conclusions, but I received no reply - unlike a few years ago when he was very helpful with my questions. I've heard that he has closed his glass studio and concentrates on other activities.
I also asked Peter Goss for an opinion on Greg's bottle and my suggestion of its maker. He agrees with the place and time, and while he was unable to say for sure, he suggested it could well be RK's work.
Trevor