Thanks for the confirmation about the photos, Mary.
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From the general "crispness" of the look of the millefiori PY weight, it could be from any of the (late) 50s, 60s or 70s periods! A photo of the base, at a slight angle, might help but it would really need a uv test with both longwave and shortwave bulbs to be sure.
If you have a longwave bulb ("blacklight") and it gives a definite green fluorescence in the dome then the weight is very probably pre-Caithness years. If the fluorescence is a "watery straw" colour or perhaps look like a "reddish green" then it is likely to be Caithness period. If there is no obvious fluoresence colour or it appears to be a "dusty pink" or a "dark straw" colour, then the Harland years are most likely.
But sometimes it is hard to say whether a non-green fluorescence colour ties in with the "rule of thumb" and therefore longwave uv alone cannot separate Caithness items from Harland pieces. That's where shortwave uv comes in - as, to the best of my knowledge so far, all of Paul Ysart's Caithness period weights, show as Blue under shortwave uv and no other period does that.
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For size of paperweights, most of us are interested in the diameter at the widest point and the height.
Even within a given maker, "standard" sizes can vary. Many of Paul Ysart's weights were a "standard" 3 1/4 inch diameter but often "only" 3 inch. Others regular sizes were 2 1/2 inch, 2 5/8 inch, 2 3/4 inch or 2 7/8 inch. In the 30s, and maybe 40s, he made several magnum sized weights ranging from 3 1/2 inch to over 5 inch diameter.