A "dark tint" is common in many Paul Ysart weights from the 1930s (& 1940s). Colour of the tint varies but purple is well known - most likely as a result of the manganese (common decolourant) in the glass batch.
Some say that the purple tint is because of too much manganese, and others say it's produced by the action of sunlight on the finished item over time (often called "sun-purpling"). One problem with the "sun-purpling" idea for Paul Ysart weights is the amount, and strength of sunlight in Scotland and the rest of the UK & Eire.
Maybe weights that spent some time holidaying in the southern states of the US could have gained a purple tan that way.