This read-only Gauge reports the number of Severely Errored Framing Seconds encountered by this DS1 interface in one specific historical 15-minute interval, the completed-interval counterpart to dsx1CurrentSEFSs. Since framing errors indicate loss-of-frame-level disruption rather than simple bit errors, historical spikes here point to specific past windows where the whole DS1's synchronization was compromised, not just individual channels. An engineer correlating a past multi-channel voice outage with switch data would check dsx1IntervalSEFSs for the corresponding interval to confirm a framing-level event rather than a bit-error-level one.