RETURN AIR FILTER, SIZE OF 680MM X415MMX115MM, AS PER RDSO DRAWING NO RDSO/ PE/ SK/ AC/ 0073- 2004 [ REV- 1 ] & CONFIRMING TO RDSO SPECIFICATION NO RDSO/ PE/ SPEC/ AC/ 0055- 2003 [ REV- 2 ] OR LATEST, SUITABLE FOR USE IN LHB TYPE AC COACHES OF DOUBLE DECKER AND SHATABDI EXPRESS