26 CFR § 301.6312-2 - Certain Treasury savings notes acceptable in payment of certain internal revenue taxes.
According to the express terms of their issue, the following series of Treasury savings notes are presently acceptable in payment of income taxes (current and back, personal and corporation taxes, and excess profits taxes) and estate and gift taxes (current and back):
(a) Treasury Savings Notes, Series A,
(b) Treasury Savings Notes, Series B,
(c) Treasury Savings Notes, Series C.