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]))
