I've had a few of these in the more typical pale amethyst colour by StrÃ¶mbergshyttan - signed and unsigned . The lack of a signature doesn't seem to be too uncommon for this design.
Edit: Ignore me - I just noticed how different the casing is on the unsigned one, and the slightly different colour. The casing on the ones I've had have been of fairly constant proportions. Your unsigned could well be another factory, in which case.