KevH uses short wave uv to help him decide between different lead contents of Ysart weights
Not quite!
What I do is check for a blue fluoresence reaction (using a shortwave uv light). If a likely Ysart / Vasart Ltd / Strathearn weight reacts blue, then (all other things looking right) I am satisfied it is either by Paul Ysart from his Caithness years or it is Vasart Ltd (1956 - 1964) or it is Strathearn (1964 to 1980).