Company Names, addresses, dates1:
P.L. Keyser, B south n 3rd East (1865)
The census for DC lists Peter L. Keyser, born 1842 in Germany, but he was listed as a physician. It's possible he put his name on a bottle for mineral water that he prescribed for his patients. It seems more likely that this bottle refers to a different P.L. Keyser, as it doesn't seem to be old enough to be from the era when people thought soda water had curative powers.
book # 348
Aqua 6.75" squat bottle with a double collar applied lip embossed:
front: P.L.KEYSER / WASHINGTON / D.C.
back: THIS BOTTLE / IS NEVER SOLD