These are the original AppleTalk DIN-3 Network Cable Couplers (to join two cables together).  This listing is for one Coupler, but may be one of two versions ("519-0300" or "519-0300-B")  Both are electrically identical, with only minor physical differences such as the icon and the the surface roughness.  One sample of each version is shown in the photographs (the Apple Logo is on the earlier 519-0300 version).

The Coupler conforms to the early DIN-3 Connector Physical Specifications.  Pin 1 on each end is wired to pin 1 on the other end.  Pin 3 on each end is wired to pin 3 on the other end.  Pin 2 is not connected on either end.  The shield on each end is connected to the shield on the other end.

Please look at the photographs and descriptions before you place your order.  You will receive either one unit of 519-0300 or one unit of 519-0300-B.  Again, there is no difference in their operation.