An electrical connector assembly comprises a first connector (1) and a second connector (1′) having a same configuration with a housing (30, 30′) and a mounting face (34, 34′) adapted for mounting onto a printed circuit board PCB (100) except for a distinguishable portion (353, 353′) provided on the housing (30, 30′) adjacent to the mounting face (34, 34′) thereof. The first connector (1) is not able to be regarded as the second connector (1′) to be mounted onto the PCB (100) due to the different distinguishable portions (353, 353′) of the first and second connectors (1,1′).