The halftrack motorcycle (I don't know what it's classed as either!) SdKFz 2 NSU Kleines Kettenkraftrad HK 101, 1938+.
Please change name: "Sd.Kfz.2" should be put as extra info, since it's only the internal army designation and therefore not part of the model name.
BSA? Girder-Fork Norton? all the WD bikes have Girder, except Matchless G3L who is (are) with Télédraulic I think it's a BSA M20 but,,,
This one has a sidevalve engine, so it's not a Triumph or a Matchless, looking at the typical BSA valve case, timing side cover and magneto drive, I also think it's a BSA M20 as built from 1937 to 1955