Milk Glass Question

I was looking for a plain white vase with some character to it and found this.
It's the size I wanted - but the weight - I could plant a small tree in it !
It's only 7" (18cm) tall, 3.5" (9cm) across the base @ its widest and 5" (13cm) across the mouth @ the widest but it weighs 1050 grams (37 oz) !
Is milk glass usually quite heavy ? Is this new ? Mid century ? Older ?
The last photo shows a square, physical indentation on the inner surface of one side. Any idea why that's there ?

 

That seller says it was made in Canada. That seller must have good eyesight - I found this on the bottom - with spy gear one might find more. I will try because that might be a date in the lower right of what looks like a circular impression.

 

Well - I found the letters I N and then an almost-imaginary M A ( at the 10 and 11 o'clock positions inside the circle in the 2nd photo) so I can accept a made in Canada attribution. I had no luck imaging any date.

 

There wouldn't be a date. They would have to make a new stamp every year it was produced.

 

Good point ! It also makes me feel better about not being able to capture more.