Internal Revenue Manual 5.5.8 provides the IRS Collection Advisory procedures for estate tax liens. This covers the §6324(a) regular estate tax lien, the §6324A special lien for estate tax deferred under §6166, and the §6324B special lien for additional estate tax attributable to farm, etc. valuation under §2032A.
The designated agent's duties are described in Reg. §301.6324A-1(b)(4), which provides:
(4) Duties of agent designated in agreement. The Internal Revenue Service will contact the agent designated in the agreement under paragraph (b)(1) on all matters relating to continued qualification of the estate under section 6166 or 6166A (as in effect prior to its repeal by Economic Recovery Tax Act of 1981) and on all matters relating to the special lien arising under section 6324A. It is the duty of the agent as attorney-in-fact for the parties with interests in the property subject to the lien under section 6324A to furnish the Service with any requested information and to notify the Service of any event giving rise to acceleration of the deferred amount of tax.
Establishing a special lien under §6324A effects a §2204 discharge of the personal liability of the executor for the amount of tax deferred under §6166.
Taking Stock in a Closely Held Business as Collateral for a Section 6324A Special Lien:
See e.g. CCA 200747019.