I always thought beneficiaries had an automatic right to the return. Guess not. Here it is from the Internal Revenue Manual https://www.irs.gov/irm/part11/irm_11-003-002.html
11.3.2.4.7 (08-29-2008)
Estates
The administrator, executor, or trustee of an estate may receive returns and return information of the estate.
Any heir at law, next of kin, or beneficiary who establishes a material interest which will be affected by the return or return information may also receive returns and return information. A material interest is an important interest and is generally, but not always, financial in nature.
Example:
Submission of a copy of a will by a beneficiary who is described in the will as entitled to "x" percent of the decedent’s gross estate, together with a statement that the decedent's return is needed to assist the beneficiary in determining whether he/she has received a proper share of the estate, would generally be sufficient to permit disclosure. The merits of an action, such as a law suit brought by a beneficiary, will generally, but not always, have no bearing on the material interest determination.
The requester must furnish satisfactory evidence that he/she is an administrator, executor, trustee, heir at law, or next of kin under applicable state law, or is named as a beneficiary in the decedent's will. Generally, written evidence such as a copy of the will, proof of relationship, or letters testamentary will be furnished to show satisfactory proof of entitlement.
See IRM 11.3.3.3, Distinction between Disclosure to Designees and the Conference and Practice Requirements, regarding representatives for estates.
Requests for returns must be made in writing.