It's m.2 ...MB key if memory serves. Made a noticeable difference in speed slapping a new drive in it.
Edit: Make a mirror of the stock drive. Made the mistake thinking a fresh install would be a good idea and now can't find some drivers.
Used a thermal pad and some electrical tape to fit a full length ssd in there. Not a lot of room for it to come loose but gave it a good shake test to make sure.
Best ever for me would be the legendary i486DX2. Memories of LAN parties with Doom, Star Wars: Dark Forces, Wolfenstein, Quake, MechWarrior, and Descent are priceless.
Back then we were bada$$ with 10BASE-T. The next major steps were T-bird and P4. From there it was corporate environment for me.