Underwriter" means any person who has purchased from an issuer with a view to, or sells for an issuer in connection with, the distribution of any security, or participates or has a direct or indirect participation in any such undertaking, or participates or has a participation in the direct or indirect underwriting of any such undertaking; but such term shall not include a person whose interest is limited to a commission from an underwriter or dealer not in excess of the usual and customary distributor's or seller's commission. As used in this paragraph the term "issuer" shall include, in addition to an issuer, any person directly or indirectly controlling or controlled by the issuer, or any person under direct or indirect common control with the issuer. When the distribution of the securities in respect of which any person is an underwriter is completed such person shall cease to be an underwriter in respect of such securities or the issuer thereof.