26 C.F.R. § 1.7520-4
Transitional rules
(a) Reliance. If the valuation date is after April 30, 1989, and before June 10, 1994, a taxpayer can rely on Notice 89-24, 1989-1 C.B. 660, or Notice 89-60, 1989-1 C.B. 700 (See § 601.601(d)(2)(ii)(b) of this chapter), in valuing the transferred interest.
(b) Effective date. This section is effective as of May 1, 1989.