The "N" *is* the SCSI version. An MFM '225 was just that, an "ST225".
I seriously don't see the "some sort of custom SCSI interface built into the enclosure" in the photos. All I see is an ST225N with an adapter connected to it to change the onboard 50 pin connector to a pair of Centronics ports on the back. Perhaps there are components under the power supply associated with the Centronics connector which supply auto-termination or something.