Can't seem to find another example of this bottle on the internet. Has the letter I and numbers 18 and either 16 or 91 on bottom with stippling. Any ideas to what it is?
My first guess would be food or household related. Beyond that, I can't say until a labeled example shows up which should be probable considering that it doesn't appear to be terribly old. For what it's worth, the neck treatment would be considered "annular rings" as opposed to ribbed which tend to run vertically.