True to our word, we cracked open a Fusion drive equipped Mac mini, and it looks like our suspicions were accurate.

The empty connector is now filled—by a PCIe cable, glued to the top of the hard drive tray.

The SSD matches the one we found in the MacBook Air 13" Mid 2013 with the same chips:

Samsung S4LN053X01-8030 (ARM) flash controller

8 x Samsung K9LDGY8SIC-XCK0 16 GB flash storage

Samsung K4P2G324ED 512 MB RAM