I was going through my FDC collection and found an anomaly. I have a couple of FDCs each with a single Flag Over Porch (FOP) plate number coil stamp...the stamp (2915C) was issued on May 21, 1996, but the cover is FDOI postmarked June 15, 1996 on the day a different FOP stamp was issued. Both are cacheted from major cachet makers.
Since it isn't the stamp that was issued that day, does that make it an illegitimate usage on the FDC? What is a cover like that called?