Weird little bottle, looks like to me its under blown, should be somewhat larger and that is, perhaps. a pontil scar but could be otherwise. A hands on inspection would be necessary for me to go farther.
Bottles like that were blown in large quantities from the early 1800's - early 1860's. It is free blown and that is a pontil scar. I have a bunch in my collection in various sizes and their application was for medicinal use. I believe the string around the neck was to hold the cork in place. They were made cheaply and rapidly, and because of that a lot of them are flawed ie they stand crooked or are misshaped. I'm going to agree with Botlguy as it certainly looks underblown.
I have several bottles with excessive glass in the heal (or bottom) zone. I usually think of it as a delayed final blow which let the parison form hang to long before the final blow was made. The still held the product. RED M