I know we've had this discussion, and since I finally have one of each in similar vintage - here they are.
To make is simpler, I've got my 115 "Wings" and the 15 "Tiffany" so I won't have to say "left/right" for you to tell them apart.
Here they are - and one big difference is the 15 won't stand up on it's own.
So it gets a soup ladle for help. The face plate/tension are slightly different, but I don't think that is due to model as my Tiffany 115 is similar to this 15.
And other views - as far as I can tell, the upper casting is the same mold.
The real difference is underneath - the base is a very different mold as are the parts.
115
15
So, the only way to tell if it's a 15 or a 115 is to look at the underneath parts LOL!!