The Social Security Administration probably has records that show who received benefits from your grandparents' accounts, but they may not be able to release that information to you.
See SSA's Commitment to Protecting Privacy through Compliance:
We cannot publicly release much of our information because it is
protected by privacy laws, the Internal Revenue code, and other
statutes. While, we can anonymize some of the information, much of it
we cannot. Our FOIA process and other transparency efforts recognize
these constraints, and we will protect privacy in all releases in
accordance with all applicable laws.
What you can do:
Note that the photocopying and research fees for making a FOIA request aren't cheap, and the information may be redacted or limited because of the Privacy Act.
You might be able to get a few clues from examining the information which is available to the public, that is, the extract from the Death Master File which is commonly called the Social Security Death Index (SSDI).
The field of interest to you is the one called "Last Benefit" or "Payment Location" which should reveal the location of the person who received the last payment or lump sum death benefit, if any was issued.
From the SSA's FAQ: Who can get a lump-sum death benefit?
We may pay a lump-sum death benefit of $255 to:
A spouse who was living with the deceased person at the time of death; or
A spouse or a child who, in the month of death, is eligible for a Social Security benefit based on the deceased person’s record.
See the question What fields are available from a Social Security Death Index (SSDI) search at different websites? for an overview of how different websites report the information they receive from the Death Master File.
The "last benefit" location might answer the question of whether your grandfather's spouse predeceased him if you knew where she lived (the "Last Residence" field), and where their eligible children lived, if they lived in a different zip code. (E.g. an eligible child might be attending college in a different town from where their parents had lived).
For more clues about who might be eligible to draw benefits, see the blog post Anatomy of a Social Security Number for a partial list of codes that might appear on records when a SSN is used as a claim number for Social Security or Medicare Benefits.
In 2015, Ancestry.com published a new collection, U.S., Social Security Applications and Claims Index, 1936-2007. This index includes information from some applications and claims made to the Social Security Administration.
Be sure to read Ancestry's description of the collection so you can see what information might be in the index, and understand the limitations on coverage. The claim types are described here (the text is Ancestry's, but I have re-formatted it for easier reading):
The most common types of claims noted include:
- Original SSN (when the original application was submitted to obtain a SSN)
- Life Claim (when a claim was made for disability or retirement benefits)
- Death Claim (when a claim was made by a surviving family member for death or survivor benefits), and
- Duplicate SSN (usually used when an application was made to replace a SS card, it may also indicate a change in SSN or that more than one
SSN was assigned).
Ancestry's instructions to order the original records only address the SS-5 (the original application for a SSN). They do not address the question of whether the Social Security Administration would release any other information about the other claim types.