The train page(in IRi I guess,not in the Govt Site) hasnt verified to which Passenger,is the slip coaches attached.I dont think its officially announced by the Rly that the Slip Coaches of Darjeeling Mail has to be attached with a particular train.Thats why while going to Haldibari,they're attached to 55749,while returning from Haldibari they're attached to 55752,because both has convenient Arr/Dep timings from NJP,when compared with 12343/44.If there is some other info there in Train Page,then I'm sure that is manually edited by some member & not attested by any Rly Official.Gussa Shant !!!