Sales of land made under deeds of trust by substituted trustees shall not convey the interest of the grantor or grantors therein, but shall be absolutely null and void, both at law and in equity, unless the substitution shall appear of record in the office of the chancery clerk of the county where the land is situated, and unless it shall so appear by being actually spread at large upon the record before the first advertisement or notice of sale shall have been posted or published; the filing for record or lodging with the clerk not being sufficient. Such substitution, however, may so appear by a separate instrument recorded as above set out in all respects, or a copy of such substitution may be recorded as above set out. Provided, however, that where the beneficiary named in the deed of trust is an agency of the United States or the State of Mississippi, a national or state chartered bank or savings and loan association, a federal land bank, a production credit association, or an insurance company, the beneficiary may substitute the trustee named therein and the trustee named in all deeds of trust held by such beneficiary in the county, by recording in the office of the chancery clerk in the county where the land encumbered is situated one (1) instrument designated as a general substitution. It shall not be necessary for the general substitution to identify individually deeds of trust affected, or the grantors therein, it being sufficient if the instrument recites the name and address of the beneficiary and declares that its purpose is to name a substitute trustee for all mortgages or deeds of trust held by the named beneficiary which are recorded in the county. All general substitutions shall be indexed by the chancery clerk in a separate book especially designated for such purpose.