I talked with Apple Forum official Apple store in
Sweden today, and a seller there agreed that it was
possible to do an after sales custom modification
of a newly delivered Retina 27 Imac (512 GB or
1 TB) flash-PCIe-SSD only with (for example) an
additional 2 TB eSATA-SSD - but that this was
likely to cost like twice as much as buying the
additional SSD yourself ... .
So the implication is that a new Retina 27 Imac
with (512 GB or 1 TB) flash-PCIe-SSD as only
storage has a free eSATA-storage slot ... .
These screws are Torx 7 - at least that was what
I used on them and got a prefect tight fit ... .