Unfortunately the packaging of HBO’s brand new release Serial Mom does not indicate the film’s aspect ratio. As our reader Kevin Adams pointed out, many online retail stores have been listing the film with a 1.33:1 aspect ratio due to the lack of information.
As correctly listed in our Film Vault however, the film is indeed presented in its original 1.66:1 letterboxed aspect ratio on this disc.

To avoid confusion and in case you planned to stay away from this release in fear you might end up with a dreaded pan & scan version, we wanted to make sure to let you all know that HBO has in fact been releasing the film in its proper aspect ratio.