I have a few hundred of these. They are more common than 18650s - at least where I source packs from.
There are a range of cells depending on age of the machine. The 506971 are in older machines, and/or smaller form factors, and the 508176 are found in newer/larger machines.
I have found ATL and Coslight brands in genuine Apple packs so far.
I have not been successful in separating individual cells without damage, but they are wired in parallel so I treat them as a combined unit. They have the added benefit of having resettable fuses already built in.
Testing so far at 2000mAh discharge, from 4.2 to 3.0 volts yields capacity around 6900mAh. From the data I can find, original capacity is 3500 per cell. Interestingly, the cells have 11.0 Wh listed on them, which indicates a capacity just under 3000mAh. I am guessing they are deliberately under rated so that they maintain their rated capacity for more cycles.
They have the added benefit that the condition of the cell can almost always be accurately determined visually.
If they are swollen, they are trash. If the 'skin' is tight, they will be good. If the skin is loose, they will be somewhere in between.
If you stack a few pairs on top of each other and wobble the 'tower' a little, you can find out very quickly if the skins are tight. No visible movement is what you want. If the tower wobbles a little, then despite their appearance, you have some loose skins.
I am pretty much convinced that these types of cells are going to be the predominant cell in the near future. They are found in every thin laptop, notebook, and tablet. The market is moving toward thinner so their use will increase. While 18650s are convenient they are not very space efficient, and certainly cannot be used in devices thinner than about 19mm.
I think that I should also note that these battery packs are also cloned by no-name manufacturers in China. Some of the packs I have pulled apart are almost as well made as the Apple ones, but others are downright scary, with no temp sensors, no resettable fuses, and shoddy soldering.