Not OP, but USDT / Tether is an example of something which exists as a proxy for something off-chain; so auditing the chain is obviously not going to work to see how much USD they actually have. Though it is possible to provably audit exactly how many USDT are in circulation at any point.

I think OP's point was more about things which exist entirely on-chain, i.e. the idea of having stock ownership tracked directly there (rather than as a proxy for real shares tracked somewhere else).

In that frame, a better example is something like the DAI stablecoin, which is backed by assets that are on-chain. So at any block, you can audit exactly how many DAI there are, exactly which assets exist to back it, and exactly what the last reported oracle prices are for those assets.