That's the Brass Clasp Detector Minion!

The Post Office believes in Brass Clasp Profiling (BCP'ing) and knows that the vast majority of fold-over manilla envelopes (FOME's) are the principal tell-tale sign. All such envelopes are presented to the BCDM (only after the customer leaves) who will grasp the envelope between thumb and forefinger and by applying slight pressure, while sliding the envelope between the thumb and finger, can deftly detect the lumpy presence of a BC (brass clasp).

Moreover, the better BCDM's can detect the presence of a BC whether it is being used (inserted through the hole and folded over) or not!