Hi Regan, welcome to the Board.
Could you please let us know the size of the weight - diameter and height. I think it is about 3 5/8 inch diameter but it would be good to confirm.
This is an interesting item and as a collector of Ysart weights, I am intrigued.
I will do some checking but for now I can say that I have seen at least one other Ysart Thistle pattern weight with the thistle formed from powdered glass. I was thinking it could have been made by Salvador, but at the time there was no proof of that.
There are several examples of Thistle pattern weights made by Salvador's son, Paul but all the ones I have seen have the thistle made of individual millefiori canes rather than powdered glass. See the example Ref PY1012 at:
http://www.ysartglass.com/PaulYsart/PYP1001.htmYour weight, with the Monart label, is the first I am aware of where the label has been used on a weight made by Salvador. However, there could be a number of reasons for that.
If you could obtain some more information about the dates when your great-grandfather was at Moncrieff's that would be very useful. As yet, the only date I am aware of is 1947 - as stated in the web page:
http://www.ysartglass.com/Ysart/YGpeople.htm - and if he was only there during 1947 then that raises a question or two about the link to Salvador Ysart - the Ysart men, except for Paul, moved from Moncreiff's (but still in Perth) to start their own company, Ysart Brothers Glass (Vasart) in 1946. There are a number of plausible theories that could answer the question(s) and the simplest one I can think of is that William Aslett was at Moncrieff's before 1947.