PCIe SSD for iMac Intel 21.5" EMC2638

Where did you get the March '14 date?

The M.2 standard is still in flux. It appears the electrical interface can be either mSATA or PCIe as such the hardware vendors have not backed one over the other method. Which is why things have stagnated a bit in producing units which could be used.

In addition, Apples implementation maybe slightly different as the standard was not ratified before Apple started production of the newer iMacs with this type of edge connector.

If you bought an M.2 PCIe SSD I would strongly recommend you check with the vendor if it's compatible if they can't tell you I would return it.

Lastly, its quite a bit of work to add it and only the newest 21.5 models even have the connector (the needed I/O parts not populated on the logic bd).