I posted below, in a response on another forum, thought it may be interesting to some here.
Bubbleback was a term used to describe the early Rolex Perpetual models, due to the thickness of the case used to house the oversized Auto-Rotor movement.
Actually there are 5 models of Perpetual movements, each with different case back, the very first model (1933 to 1935) was not looking like a 'bubbleback' case but more like the bottom of a 'coke can'
Here is a picture of the earliest and rarest Rolex Perpetual (a DIDACTIC 1933) versus a 'BUBBLEBACK' 1949
You can see the 'DIDACTIC' is very less 'Ovetto'
I own both of these