The retirement board may authorize a trust company or a trust department of any state or national bank authorized to conduct the business of a trust company in this state or the Federal Reserve Bank of San Francisco or any branch thereof within this state, to act as custodian of any securities invested in by the retirement board. Any such bank or trust company may be authorized to collect the income from such securities or the proceeds of the sale thereof for the retirement board, and deposit said income or funds in the account of the retirement system. The compensation of such bank or trust company for such custodial services shall be fixed by agreement and shall be paid in the same manner and from the same funds as are other costs of administration of the retirement system. Securities of the retirement fund held by the custodian bank or trust company may be registered in the nominee name of the custodian or of the retirement system. The custodian bank or trust company shall make such disposition of the securities as the retirement board shall authorize. All such securities are at all times subject to the order of the retirement board.