Complex sentence: Censored mail can be identified by various postmarks, dates, postage stamps and other markings, along with an adhesive seal, usually bearing special ID markings, which were used to reclose the envelope after inspection.
Split into simpler sentences: Censored mail can usually be identified by various postmarks, dates, postage stamps and other markings. These covers (envelopes) often have an adhesive seal, usually bearing special ID markings, which were applied to reclose and seal the envelope after inspection.