Section 6. The preceding section shall not apply to any corporation doing business under its true corporate name, nor to any partnership doing business under any title which includes the true surname of any partner; nor to any association which has complied with sections five and six of chapter one hundred and fifty-nine; nor to any partnership, joint stock company or association the business of which is conducted by trustees under a written instrument or declaration of trust, provided that the names of such trustees with a reference to such instrument or declaration of trust shall be filed as provided in section five, nor to any limited partnership organized or qualified under chapter one hundred and nine doing business under its true partnership name if such name contains without abbreviation the words “limited partnership”; nor to a limited liability company or limited liability partnership which is doing business under its true name and which has registered or qualified with the office of the state secretary.