Rockwell.... but I've never seen it listed or referred to by that, I've only noticed the Rockwell logo on some of the parts.
I suspect it is better known as a GM axle which makes a lot of sense because I've never ever seen one under anything else (from the factory).

If you can't get a wrecking yard to find something by application you need to find a real wrecking yard...

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-