Old milk bottles often had huge embossed letters on the bottom of the bottle. I believe each dairy had a different letter.
Found this page, too, about letters on bottles by: K. Johnson
A.....................The letter "A" is often merely a mold letter. If it is an abbreviation for a glass factory, it could stand for one of several companies. One possibility: Agnew & Company, Pittsburgh, PA (c.1854-1894+). See "A & CO." mark. Another possibility would be Adams & Company, Pittsburgh, PA (1861-1891). Originally Adams, Macklin & Company (1851-1861), they did manufacture at least one type of fruit jar circa 1866, lettered "Adams & Co./Manufacturers/Pittsburgh,Pa." on the front. However, the great bulk of their glass production seems to have been pressed pattern glass and high-quality tableware and novelties, especially throughout the later years of their operation. In 1891, they joined the merger known as the United States Glass Company. Note: If the bottle is machine-made, the letter "A" would indicate a much more recent company, perhaps Arkansas Glass Container Corporation, Jonesboro, AR (1958-to date).
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