Did some checks and finally got to know about it...the loco displays 3rd CP provided... that is the additional compressor provided.. this is not so common though and as per what I could find there were 2 WAP4 to which this was done by ELS CNB on trial basis for some additional air requirement on Duranto trains...in fact ELS CNB lists this (providing 3rd CP) as one of its innovations/accomplishments in the white papers...