..., there does not seem to be much info about Selkirk about, I cannot find anything about William Lowery ...
Correct - not much on Selkirk online: it is my next project, and consequently I would very much appreciate if you'd allow me to use your images of this one-off weight, which will not show up in the catalogues. I cannot tell you, however, how common one-offs were for Selkirk.
Now on the artists: they are named in the catalogues from 1990 to 2004 (last full catalogue) with the exception of Chris Dodds (for his non-paperweight designs). Only in 1999 to 2002, however, certain paperweights were attributed to individual artists.
The artists I find mentioned (apart from Peter Holmes, of course) are:
Chris Dodds 1988-1993
David McGregor 1990-2004
James Brown 1990-2001
William Lowery 1990-1998
Philip Holmes 1991-2002
Martin Coyle 1994-2004
Kenneth Scott 1996-2003
James McBeath 1998 only
McDiarmid Fiona 2002-2004
So William Lowery had left by 1999, when designs were first attributed to individual artists - but 2 of his older designs were still in production and listed: Tiger Lily and Windswept (both in several colourways).