Thanks for the additional photos. I feel pretty strongly that the keystone marking contains the letter "M" - your last picture is clear.
That would be the manufacturer mark of Metro Glass Bottle Co. of Jersey City, NJ which they used from 1935 to 49. (according to Bottle Makers and Their Marks) Additionally, the 96 marking has been used by Metro, according to a document on Permit Numbers that is found on Bill Lockhart's website.