OK, got the new tool set, and yes, it really is actually a T5 screw, you just need a really sharp driver. My existing T5 driver was getting a little dull on the tip. So make sure you have a really new/precision T5 driver/bit to use (and again I'll point out that ifixit failed to list this tool in the list of needed tools at the top).
I'm sure you've long since figured it out, but for the benefit of anyone else reading this, a P5 is not a T5. It's a different screwdriver. The driver for the screws on the outside of the case will not work on the screw holding the SSD. I'm not sure why they left it off in the list of tools at the top.
I was needing to send my mid 2014 15" MacBook Pro in for a keyboard repair, and my company IT department shipped me a loaner of the same model except that it had a smaller SSD in it than mine, and said I should swap the SSDs between the two and then send mine back with the smaller drive in it to get fixed. I've been unable to swap the drives because neither mine nor the loaner (which are both the same model) appear to have a T5 screw holding the SSD in place. Both of them appear to have a T4, not a T5 (whatever it is is smaller than a T5 anyway, so I'm guessing it's a T4). So now I'm on hold until I can find a T4 screwdriver.
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