RE: Why the different roof?

The FtDDM&S car is an ex B&M 72000 series car (4 door supports/guides).

I am suspecting the WAG is an ex-71000 series (I can’t see the door area to confirm)

They has different roofs “as built”.

See here for more details

            https://www.bmrrhs.org/box_71000_series/

Ken