Just the M key.
One thing, I first tried the cheap PCIe adaptors and my SM951 started heat throttling easily, if you don't want this happening buy a heatsink or go for a more robust solution like Angelbird Wings PX1 or Aquacomputer kryoM2.
Adorama eBay store has the best price on Wings PX1.
Another quick question, if I get a M+B key SSD (like this one: https://www.amazon.com/gp/product/B07C8XDV11/ref=ox_sc_act_title_1?smid=ATVPDKIKX0DER&psc=1') will there be any compatibility issues if I place it in a M keyed adapter for the mac pro?
Edit:
Copied the wrong link