J1939 type 2 female connectors are physically backwards compatible, while type 1 female connectors only fit type 1 male sockets. The type 2 connector was designed for the SAE J1939-14 standard, which adds support for 500K bit rates. The purpose of "blocking" type 1 connectors is to ensure that older hardware (presumably using 250K bit rates) is not connected to type 2 500K bit rate J1939 networks. Specifically, the physical block is through a smaller hole for pin F in the type 2 male connectors.
Note that the J1939 deutsch connector comes in two variants: The original black connector (type 1) and the newer green connector (type 2), which started getting rolled out in 2013-14. Green type 2 J1939 adapter cables can be used on both black and green connectors as they are backwards compatible.
