def is_other_or_unknown(df):
    """Return if vehicle is other/unknown, per NHTSA convention."""
    yr = df['YEAR']
    body = df['BODY_TYP']
    tow_veh = df['TOW_VEH']

    return ((yr.between(1975, 1981) & (body.between(35, 42) |
                                       body.isin([44, 45, 99]))) |
            (yr.between(1982, 1990) & (body.isin([13, 14, 42, 52, 73, 77,
                                                  80, 81, 82, 83, 88, 89,
                                                  90, 99]))) |
            ((1991 <= yr) & (body.isin([12, 13, 23, 42, 65, 73, 90,
                                        91, 92, 93, 94, 95, 96, 97,
                                        99, 98]) |
                             ((body == 79) & tow_veh.isin([5, 6])))))