Lenovo ThinkStation P7 Manual page 14

Then, when looking at the inner side of the SATA drive bay, you will also
find two mounting channels on the chassis and a card connector socket
directly beneath on the motherboard:
In order to mount the M.2 holder, insert the mounting tabs into the channels, align
the interposer card edge connector with the socket as shown below, and press the
whole assembly straight down until you feel it lock into place.
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