They are special in that the SSTF can't sell them to any third party but the Treasury pays interest the same as anyone else. The "special securities" by law can be redeemed at anytime prior to their maturity date which is something that public buyers can't do with theirs.
That is true for all of the bonds issued by the Treasury not just the ones they sold to the Social Security Trust Funds. Bonds are sold by the Treasury with the full faith and trust of the US Government and if they default on them then the costs to manage the government debt will exploded.
Please explain this:
The social security trust fund has a positive balance of 2.7 trillion dollars. Their positive balance has been increasing throughout the Obama years. Hardly a Ponzi scheme.