... i used a black light and nothing glows ...
When thinking about whether a weight may be a fake py example, a blacklight (i.e. longwave uv) will not always help. This is because the longwave uv reaction of the fakes is much the same as for genuine PY weights from his 1970s Harland period - which many people will see as showing "no reaction". Some people may see a very pale "orangey" colour, but again this will be much the same for the fakes and the Harland weights.
Since your weight shows "no reaction" but, as has been said, it is definitely a genuine PY weight, it is from the Harland years as opposed to the Caithness Glass period. The blacklight (longwave) reaction for Paul Ysart Caithness weights is yellow / orange (but can vary in strength of the colour). The uv reaction can also vary in strength depending on whether there is any ambient regular light and also the distance of the lamp from the surface of the weight.Edited to add:
When you said "nothing glows" was I correct in thinking you meant "no uv reaction"? Or is there a yellow / orange colouring?