(a) Obligations of or guaranteed by the United States. Interest-bearing direct obligations of the United States, or obligations fully guaranteed as to principal and interest by the United States, may be deposited in the construction reserve fund in lieu of cash, may be purchased with cash on deposit in the fund, or may be substituted for securities or commitment to finance in the fund, subject to the provisions of paragraph (b) of this section.

(b) Other securities. In cases where the taxpayer desires to deposit any securities in the fund in lieu of cash other than those of or guarantees by the United States or to purchase such other securities with cash on deposit in the fund, or to substitute such other securities for securities or commitment to finance in the fund, the taxpayer shall make written application to the Administration and shall not consummate the transaction until the written consent of the Administration shall have been received. The application shall describe the securities fully. Every approval by the Administration of such application shall be conditioned upon agreement by the taxpayer forthwith to dispose of such securities upon subsequent request by the Administration. Immediately upon the purchase of any securities for deposit in the fund, the taxpayer shall advise the Administration, giving the date of purchase, a description of the securities, and the price paid therefor (net, brokerage and other charges, and gross). Ordinarily, the Administration will not approve the deposit in the fund in lieu of cash, or the purchase with cash on deposit in the fund or the substitution for securities in the fund of securities not actively traded in on exchanges registered under the Securities Exchange Act of 1934 (15 U.S.C. ch. 2B), or securities which are not legal for investment of trust funds. Whenever the Administration approves the substitution of other securities for securities in the fund, such substitution shall be effected only upon or after the deposit of the substituted securities into the fund.

(c) Cash. Cash may be substituted for amounts which are on deposit in the fund in any other form.

(d) Devalued securities. In the event the Administration determines that the market value at any date of any securities in the fund has decreased to a figure which is less than 90 percent of the market value at the time of deposit into the fund, then within 60 days after the taxpayer receives notice of such determination the taxpayer shall (except as otherwise provided in this paragraph) deposit into the fund cash or securities in an amount equal to the difference between the current market value of the devalued securities and the market value of such securities at the time of their original deposit. However if any securities in the fund are valued at the time of their deposit at less than the market value of such securities at the time of their deposit the taxpayer shall be required to deposit only an amount equal to that portion of the difference between the current market value of the devalued securities and the market value of such securities at the time of their original deposit which bears the same ratio to such total difference as the amount at which the securities were valued at the time of their deposit bears to the market value at the time of such deposit.