I found this piece today while walking the railroad tracks looking for insulators. I cant say that I've ever seen a milk shaped like this one before, anyone familiar with something this?
This is what is referred to as a "cream top". In the days before homogenized milk and skim milk you got whole milk that had been pasteurized. Whole milk naturally separates into cream and milk, with cream floating on top. Someone figured how much cream would float on top of a regular quart of milk and designed the neck in such a way as to indicatd approximately where the cream ended and the milk began. Silverwoods dairy used this feature quite extensively in advertising their milk.
But you have only half of the story. A small "S"curved spoon was used to scoop out the cream and these are the other half of the story - also quite hard to find nowadays. Any milk bottle collector can show you what these spoons look like.
Homogenization was developed to spread the cream throughout the milk so that it wouldn't separate. Skim milk has the cream removed or partially removed as in 2%.