27 C.F.R. § 70.28
(2) Examples. Examples of summonses referred to in paragraph (a)(1) of this section are:
(b) Numbered account (or similar arrangement). Under 26 U.S.C. 7609(c)(2), a summons issued solely to determine the identity of a person having a numbered account (or similar arrangement) with a bank or other institution is excepted from the requirements of 26 U.S.C. 7609. A “numbered account (or similar arrangement)” under 26 U.S.C. 7609(c)(2) is an account through which a person may authorize transactions solely through the use of a number, symbol, code name, or other device not involving the disclosure of the person's identity. A “person having a numbered account (or similar arrangement)” includes the person who opened the account and any person authorized to use the account or to receive records or statements concerning it.
(26 U.S.C. 7609)
[T.D. ATF-301, 55 FR 47610, Nov. 14, 1990]