Yes there are also "genuine" things on the CB's balance sheet. But notice how small they are, except in some weird cases (like the SNB, the Swiss National Bank, which hoards a ton of equities).

My overall point was that the central bank, the public pensions system and the treasury are all a part of the federal government. When they "trade" with each other and maintain separate balance sheets that really means very little. What's the large scale difference between a tbill and base money if the Fed maintains an open market operations center and guarantees to trade them in unlimited quantities (repo and reverse repo) to keep them priced 1:1 to enforce their monetary policy? I mean I know you cannot take a tbill to a drugstore, but you can trade it back to a spendable private bank account balance in an instant, and you will always be able to.