SSDs were not yet the ubiquitous default, not by far. Many laptops shipped with HDDs or SSHD (solid state hybrid drive) for faster storage. The most common SSD fit in the slot to replace these so they were 2.5" and SATA. Cheap NVMe as a category started to emerge that year but only H2 of it especially with the Intel 600p. Also, most laptops could only run them at x2 because the On Package Interconnect between the CPU and the PCH was set to 2GT/s instead of 4GT/s for better battery life. This made total sense: SATA only could use less than 1GT/s anyways so why bother? The other potential bandwidth hog was Thunderbolt 3 which did appear on Skylake laptops but most laptops used the Intel JHL6240 which only used two PCIe lanes instead of four once again for power consumption reasons and so they thought oversubscribing the 2GT/s link with SATA and Thunderbolt is okay. Even to this day the only semi-common high bandwidth Thunderbolt peripheral is the external SSD.
The next year, in the ThinkPad T470 Lenovo will start selling what OP hacked here: it had a kit which put a M.2 key M slot inside the HDD bay and connected with the appropriate cable.
I guess Lenovo has a history not unlocking full PCIe slots. Way back in the day, I found a footprint (solder pads) for a mini PCIe slot on my Thinkpad's motherboard. Well, I did what any reasonable person would do and soldered on a connector. It worked, surprisingly enough.
I tried swapping the Wifi card to that slot and it worked. This is before m.2 SSDs became popular, so I didn't exactly have much use for the slot, it just sat empty afterwards. Or until I bricked the laptop by trying to flash coreboot, IIRC.
The last two images are missing for me, do they work for anyone else?
Also interesting to know that there was a Thinkpad 13 - I'd been ignorant of that model despite purchasing a T460 of the same generation. (Although there's also the classic Thinkpad X13, the newer Thinkpad Z13, and the Thinkbook 13 so I don't feel too bad.) Here's some more information: https://www.mobiletechreview.com/notebooks/Lenovo-ThinkPad-1... (including an unusual looking internal layout what with the looong single heatpipe and empty space in the corner).
SSDs were not yet the ubiquitous default, not by far. Many laptops shipped with HDDs or SSHD (solid state hybrid drive) for faster storage. The most common SSD fit in the slot to replace these so they were 2.5" and SATA. Cheap NVMe as a category started to emerge that year but only H2 of it especially with the Intel 600p. Also, most laptops could only run them at x2 because the On Package Interconnect between the CPU and the PCH was set to 2GT/s instead of 4GT/s for better battery life. This made total sense: SATA only could use less than 1GT/s anyways so why bother? The other potential bandwidth hog was Thunderbolt 3 which did appear on Skylake laptops but most laptops used the Intel JHL6240 which only used two PCIe lanes instead of four once again for power consumption reasons and so they thought oversubscribing the 2GT/s link with SATA and Thunderbolt is okay. Even to this day the only semi-common high bandwidth Thunderbolt peripheral is the external SSD.
The next year, in the ThinkPad T470 Lenovo will start selling what OP hacked here: it had a kit which put a M.2 key M slot inside the HDD bay and connected with the appropriate cable.