Been a lot ninja’ed while I was blathring. But FWIW here’s me trying to put it all in one coherent answer . ,
Completely setting aside for a moment the distinction between you and your brother.
The piece of paper certainly DID at one time serve as a record of your ownership. At some time back in WAG 1970s, every corporation in America switched from paper certificates as evidence of ownership to a central share register maintained by the corporation. The actual clerical work was commonly outsourced to a brokerage, but that register became the one and only official list of who owned how many shares. When that centralization happened to those shares, the paper stock certificates were cancelled as financial instruments and the fancy pieces of paper became a neato souvenir of a bygone swell era. Nothing more.
Now at that time you would either have been notified by mail that your ownership record was in the central registry, your certificates are null & void, and if you ever wanted to sell your shares contact your own brokerage. OR, as others have said, it’s common at these kinds of big reorganizing events for companies to buy out great numbers of individually tiny shareholdings to simplify their work. IOW, we’ll buy your shares at next month’s end of month share price if you want; just return this form and we’ll mail you a check for your shares. Either way your certificates are now souvenirs, not financial documents.
Something similar would happen at any merger / buyout. The shareholder of record at the central registry would get snail-mail offering either to buy the old shares outright, or you’ll get X shares of the new company for every Y shares you hold in the old company, or some combo of cash and new shares. Whichever option you choose the old shares legally disappear at the merger. Leaving all the old company shareholders with some combo of cash and new shares in the new company at that new company’s central share registry. And perhaps leaving some old company shareholders with souvenir pieces of paper from the old company.
Switching back to reality with both brother & you.
You’d now need to trace the history of when that company switched to central share repository, what you or (more likely) your brother did way back then ~50 years ago to keep or sell back the shares. Then what, if anything, else he ever did with his centrally recorded shares from then forward until either the merger or his death, whichever came first.
If the merger came first, you’d need to find out what option he chose in the merger. If he kept shares in the new company, then you need to trace whether he ever sold them before his death. If not, those should have transferred to you during the estate / probate process. If his brokerage account was POD you, the fact you didn’t get anything through that then says there was nothing to get; the shares had been sold somewhere along the way. If it went through probate, there’s the issue that you’d need to know to contact that brokerage to tell them about the probate. People forget about long-dormant accounts all the time. The heirs never discover them either.
If he did still hold shares in something at his death and they remain unclaimed long enough (~10-20 years depending on the state), they go to his state’s unclaimed property department.