I don't know the date of your Rock Gate Farm bottle either. I had to blow it up big to discover that it was from Mt. Kisco. I see that there is embossing on the heel that may speak to date.
Today you can drive on Rock Gate Farm Road in the Bedford Hills. If you look at the bottom right of this July 20, 1950 Issue of The Putnam County Courier you will see a large ad for Rock Gate Farm Dairy. The ad shows their gate & calf insignia, cites their "milk bar and restaurant," and proclaims that their milk is "available now in handy, disposable cartons--so convenient." There is a "Note: Rock Gate milk and cream are still available in bottles, too, and home deliveries will continue to be made in bottles exclusively."
This is the first I've noticed a "Third Quart" milk.
"In addition to 12 ounce bottles one will also find 10 ounce and third quart (10.67 ounces) cafe milk bottles. Some bottles will be embossed 3/4 PINT which was the same as 12 ounces. In California, 10 ounce milk bottles were not common. The 12 ounce cafe bottles were most common during the period round bottles were in use and in square bottles the third quart cafe bottles became more common. States often regulated what size bottles could be used to sell milk in the state. Some towns taxed drinks over a certain price, such as 10 cents. In those instances smaller size servings were more popular to avoid the tax. Click here for a picture comparing a pint, 12 ounce, 3/4 pint, 1/3 quart and 10 ounce milk bottle." From.