If the Social Security fund was invested in the stock market...?

[A HERETIC I AM]

36. No, by definition they are anything BUT IOU's

And you are wrong to categorize them as having "No marketable value".

They are called "SPECIAL ISSUE" because they have two important provisions available only to the SS trust funds;

1). They are redeemable on demand at face value, unlike regular US Treasury Bonds which are only redeemed at face value upon maturity (Often referred to as a "PUTABLE Bond", as they can be "put" back to the issuer at any time), and

2) They have a fixed value as they do not trade on the open market. THIS is why I say you are wrong to say they have no marketable value.

Sure, you and I can't buy one of these securities, but they most certainly have value.

The primary difference between these securities and an "IOU" is that they are BONDS that pay periodic interest payments TO THE TRUST FUNDS

By law, income to the trust funds must be invested, on a daily basis, in securities guaranteed as to both principal and interest by the Federal government. All securities held by the trust funds are "special issues" of the United States Treasury. Such securities are available only to the trust funds.

In the past, the trust funds have held marketable Treasury securities, which are available to the general public. Unlike marketable securities, special issues can be redeemed at any time at face value. Marketable securities are subject to the forces of the open market and may suffer a loss, or enjoy a gain, if sold before maturity. Investment in special issues gives the trust funds the same flexibility as holding cash.

Also;

Why do some people describe the "special issue" securities held by the trust funds as worthless IOUs? What is SSA's reaction to this criticism?

Money flowing into the trust funds is invested in U. S. Government securities. Because the government spends this borrowed cash, some people see the trust fund assets as an accumulation of securities that the government will be unable to make good on in the future. Without legislation to restore long-range solvency of the trust funds, redemption of long-term securities prior to maturity would be necessary.

Far from being "worthless IOUs," the investments held by the trust funds are backed by the full faith and credit of the U. S. Government. The government has always repaid Social Security, with interest. The special-issue securities are, therefore, just as safe as U.S. Savings Bonds or other financial instruments of the Federal government.

Many options are being considered to restore long-range trust fund solvency. These options are being considered now, well in advance of the year the funds are likely to be exhausted. It is thus likely that legislation will be enacted to restore long-term solvency, making it unlikely that the trust funds' securities will need to be redeemed on a large scale prior to maturity.

The above quotes can be found here;
https://www.ssa.gov/oact/progdata/fundFAQ.html#&a0=6

It is indeed likely that you and I are in agreement on many issues, but US Treasury Securities are a particular fascination of mine and I am fairly well versed in their structure, issuance and pricing. Suggesting the securities held by the Social Security Trust Funds are IOU's is again, inaccurate and simplistic and is frankly an insult to the integrity of these instruments.