I went further in every mac and got this:
"Apple does not intend for end users to upgrade the SSD in these models themselves, the company even has used uncommon "pentalobe" screws -- also called five-point Torx screws -- to discourage access. However, access is straightforward with the correct screwdriver, the SSD is simple to access, and upgrades are not blocked in firmware, either. There are two significantly different SSD designs for these models, though.
There is one 6 Gb/s SATA-based SSD for the "Late 2012" and "Early 2013" 13-Inch Retina MacBook Pro models. Unlike the 15-Inch Retina Display MacBook Pro, the SSD in the "Late 2012" and "Early 2013" models is mounted in a small "drive caddy" that can hold a 5 mm or 7 mm tall SSD (or hard drive), in lieu of the small proprietary SSD module that Apple uses.
The proprietary PCIe 2.0-based SSD in the "Late 2013" and subsequent models is limited to a smaller "blade" option, though. By default, the "Late 2013" and "Mid-2014" models negotiate a x2 PCIe connection, but in testing, OWC discovered that when a "blade" SSD from a Cylinder Mac Pro was installed in a "Late 2013" or "Mid-2014" model, it "negotiates a x4 PCIe connection versus the stock cards, which negotiate a x2 PCIe connection," which means that this newly transplanted SSD was substantially faster than the stock one. The "Early 2015" models support a x4 PCIe connection by default."
I don't know if this helps.