indication may be formed by one or both opaque blocks 16 18 since an opaque block cannot be shown in the drawings the boundaries of the opaque blocks are indicated with dashed lines which it will be understood are not actually present on the face 14 and boundary demarcation line 17 arranged on the face 14 to provide a visible demarcation between jacks for different catheters each indication 16 17 18 has a catheter label area 16 a 17 a 18 a on which a catheter identifier is provided the identifier may for example be selected from the well known u201c ra u201d u201c rv u201d etc notations which clinicians use to indicate locations within the heart and so provides an intuitive indication of which catheter to connect each indication 16 17 18 also has wire label areas shown generally at 16 b 17 b 18 b but not necessarily having visible boundaries on which wire identifiers for example numerals u201c 1 2 3 u201d corresponding to the manufacturer provided designations are provided each in registration with an individual hole 13 an indication block 19 also is provided which delimits the non grid jacks 5 which are provided specifically to accept jack plugs from stimulation electrodes and are labeled u201c s u201d and u201c s u2212 u201d identifying the polarity of the stimulation pulse each overlay is provided in the block 20 with the unique identifying label 15 such as an alphanumeric u201c a 1 u201d thus as can be appreciated from the illustration of the overlay 9 presented in fig2 a this arrangement provides a visible indication by which catheter.