1) A model that had decals on a blue plastic box (instead of printing on the plastic), with a metal mesh speaker cover, an adjustable rod, and a square,blue,open coil.
2) Another model, also blue, had logos etc printed onto the blue plastic box, plastic speaker grill integral to the control box, a non adjustable rod and a small round, open coil.
3) Finally, one similar to the second one mentioned, but with a grey control box and a round, black, closed coil.
When were each of these made? Any history would be appreciated!
Many thanks, Jennison