The sources I could find say that they didn't replace the computers accessing the data. They are still IBM Series/1 [1] machines. So whatever is being used, there has to be some kind of floppy (or tape, etc) emulator in front of it.

It's likely the floppy drive in question was an IBM 23FD "Minnow"[2]. The capacity and timeframe matches up.

[1] https://en.wikipedia.org/wiki/IBM_Series/1

[2] https://historyofinformation.com/detail.php?id=885