31 C.F.R. § 309.8
Tenders should be submitted on the printed forms and forwarded in the special envelopes which will be supplied on application to any Federal Reserve Bank, or Branch or to the Bureau of the Fiscal Service, Washington, DC 20226. If a special envelope is not available, the inscription "Tender for Treasury Bills" should be placed on the envelope used. The instructions set forth in the public notice announcing the offering should be observed with respect to the submission of tenders. Banking institutions generally may submit tenders for account of customers provided the names of the customers are set forth in such tenders. Others than banking institutions, will not be permitted to submit tenders except for their own account. Tenders from incorporated banks and trust companies, and from responsible and recognized dealers in investment securities will be received without deposit. Tenders from all others must be accompanied by a payment of such percent of the face amount of the Treasury bills applied for as the Secretary of the Treasury may from time to time prescribe: Provided, however, That such deposit will not be required if the tender is accompanied by an express guaranty of payment in full by an incorporated bank or trust company. Forfeiture of the prescribed payment may be declared by the Secretary of the Treasury, if payment is not completed, in the case of accepted tenders, on the prescribed date.
31 C.F.R. §309.8