SSD OEM Exact Model, Part Number and Manufacturer in MacBook Pro (Mid 2012)
It is a Samsung 830 (SM512E) for both the Retina and non-Retina Mid-2012 Macbook Pro. 

Ref: 


Apple Forums 
Anandtech 
MacRumors


Edit: Apple has also begun to use the Sandisk X100 in Retina Macbook Pros.