Part 1 - When Blog Post Link confirms North Bengal Exp renamed as Darjeeling Mail then why you confused?
Via Manihari (1950) was longer with mix of Ferry+MG. North Bengal has to depart early to reach NJP at morning!! 1 April 1964 Via Farakka, NB evening DEP & in 1965 converted to Darjeeling Mail on Popular Demand!!
Attached TT shows 1961 & 1975 while changed happened in 1964-65. Still 43 N.B. & Darjeeling are linked!!