It's gorgeous, Nic.
All I can shove in are a few random thoughts.
S&W did a range of iridescent pieces with lion head prunts, called Caerleon, in 1929. (ref. Lesley Jackson's 20th Century Glass, p.196. But I think your piece looks a bit thick to be this - however, I've never handled a piece.
It does look fumed.
Other makers I know who have done this sort of fumed black background (with and without coloured inclusions) are Bertil Vallien for Kosta Boda and Will Shakespeare. (I can get pics of the last two if you'd like).