(FORTUNE Magazine) – Shearson Lehman Hutton has invented a new vehicle with its latest security package -- called an unbundled stock unit -- but a number of companies want someone else to take it out before them for its first spin. Brave test drivers: Pfizer, Dow Chemical, Sara Lee, and Shearson's parent, American Express, which will offer to exchange up to $5.7 billion in outstanding stock for the USUs, as they're known, perhaps in late February. Shareholders of each company will have the opportunity to exchange their ordinary shares for a package of three securities. For each share, they would probably get: -- One bond, which will trade at a deep discount to its face value. It will pay annual interest equal to the stock dividend at the time of the swap. -- One special warrant that allows holders to buy ordinary stock at a price equal to the maturity value of the bond plus the value of one preferred share. This will allow investors to capture all the appreciation in the stock's price. -- One preferred share. This will pay incremental dividends equal to the difference between the stock dividend at the time of the swap and the dividends paid in subsequent years. The USU is Shearson's attempt to open to investors the hidden value of companies. Stocks can trade at prices well below a company's breakup value. Knowing this, raiders like to take over a company and take it apart. Asks Ron Gallatin, a managing director at Shearson and the chief architect of USUs: ''Do you really have to unbundle the company to get at the assets and increase shareholder value?'' USUs could push up share price. The main reason: Corporations would save taxes. For one thing, the interest paid on bonds is deductible, whereas dividends are not. Capital gains from share-USU exchanges are taxable and therefore probably only such tax-exempt institutions as pension funds will participate. It's still uncertain whether USUs will catch on. Some chief executives are dubious. Says David Roderick of USX: ''Things that investment bankers dream up are rarely good for anyone but the investment banker.'' W.E.S.