N193DN operating DL123 EDI-BOS diverted to SNN yesterday and is still at Shannon scheduled to operate SNN-JFK tomorrow as DL9960. N1603 was dispatched early this morning from JFK to SNN and operated SNN-BOS as DL9887, and then BOS-JFK as DL9961. That's the one that wound up at Terminal E. They left SNN within the time when Shannon's preclearance operation is still open, though not knowing specifically how that works I can't say whether or not it's feasible for a diverted flight to take advantage of that facility. If not, that'd explain why they wound up at E. (I.e., does SNN preclearance have the capacity to handle an unexpected extra 767's worth of passengers who weren't even supposed to be in Ireland but for the diversion? Some other technical issue at SNN?) Alternatively, DL123 from Edinburgh is supposed to get in at 2:05 PM and the replacement flight from SNN got in at 5:00 PM or so; there's only so many widebody-capable gates at A, and while I'm not familiar enough with Delta's Boston schedule to know the answer, is it possible that there just wasn't free space for an extra 767 at A? (After all, being a replacement it apparently headed back to JFK empty if the 99XX-series flight number is anything to go by, so it's not like it'd matter if it left from E once it'd dropped the diverted passengers off.)