Jacob 321 UG is a neoprene sealing ring sized to adapt an M25 threaded cable gland to a PG21 mounting hole, supplied in packs of 50. The elastomer ring compresses between the gland body and the enclosure wall to maintain the IP rating at the retrofit interface. It is intended for installations where existing PG-threaded knock-outs are re-used with metric gland stock. Typical application is upgrading control cabinets and machine enclosures from PG to metric cable entries without redrilling.