1936 DDO#2 and MD
Decided to snap this photo for some newcomers to help differentiate between machine doubling and hub doubling.
Arrows in black point to the distinct notching left by hub doubling, whereas the white arrows denote where machine doubling is present. Note the distinct notching versus the flat, shelf-like doubling in the MD areas.
A picture is worth a thousand words, and I think this example has captured the best of both worlds for newcomers to be able to compare side by side.