Ocean Technology Systems MK III 3 Diver Intercom Underwater Breaker Other differences include: 1) The Locking Collar Other differences include: 1) The main exhaust body of the BandMask® 28 is part of the frame itself and uses a #545 041 main exhaust cover 2) no comfort insert is required on the “28” 3) the face ports for the 18 and the 28 differ slightly in size 505 100 Medium Neoprene Neck Ring Assembly (For SL 27) Military and Professional divers The ability to connect two boxes together gives even greater versatility to the MK III 3 Diver Box by allowing up to 6 divers to talk simultaneously with a surface tender