When S&L depositors are invited to buy shares

For people used to getting nothing more from their savings-and-loan association than a monthly statement or an invitation to open a Christmas Club, the most recent letter to land in their mailbox may have been surprising and perhaps confusing.

Customers of some S&Ls are being invited to become stockholders. Every month, more and more savings-and-loans and savings banks are ''going public,'' joining hundreds of other thrifts that are already owned by the investor public. Instead of being ''owned'' by the depositors, these ''mutual'' institutions are raising capital by selling shares. Through the middle of August, says Jonathan Gray, banking analyst with Sanford C. Bernstein, a New York brokerage firm, some 21 S&Ls sold shares to the public this year. These offerings raised more than $1.6 billion.

In many instances, the first people to be offered shares of the new issue are the S&Ls' depositors. But many of these people have never done anything more daring with their money than keep it in a savings account, one of the new money market deposit accounts, or maybe a money market mutual fund. Now they are being asked to buy stock.

Generally, Mr. Gray says, an investment in your local S&L is not a particularly bad one. In fact, the industry as a whole looks pretty good right now, although there are numerous exceptions. He believes short-term interest rates - the ones that can wreak havoc on an S&L's earnings when they go up - are unlikely to rise from their current 9 percent level.

At the same time, thrifts are getting 12.5 to 14 percent for their fixed-rate mortgages, giving them a ''spread'' of three to four percentage points, about as good as they've had for some time. And next year, Gray believes, short-term rates may get down around 7 percent, which would make S&L stocks even more attractive. A drop in short-term rates means S&Ls do not have to pay as much for their deposits, helping to increase profits, which are partly passed along to shareholders.

As the money from the stock issues builds up, Mr. Gray believes, S&Ls will also make more commercial loans, something federally chartered thrifts were prohibited from doing before last year, when Congress changed some of the banking laws. If they are good-quality loans, they could provide more profits for the S&L.

But not all S&Ls - perhaps including some of those that invited depositors to become shareholders - are automatically a good investment, Gray cautions. So if receiving one of these letters gets you interested in the idea of buying S&L stock, he suggests some comparison shopping with other S&Ls that are also selling stock.

Don't worry about not getting a better deal because you buy stock in an S&L where you don't have deposits. Depositors do not get any better price on an initial stock offering than anyone else; they just learn about it sooner than the rest of the general public, though perhaps no sooner than the local brokerage community.

Gray looks at three general criteria to help him decide if an S&L or savings bank might be a good stock purchase:

1. The yield on the institution's loan portfolio. If it is 11 percent or more , that's one good sign. If it is 10.5 percent or less, it may be a sign of too many low-interest loans and mortgages in the portfolio.

2. Operating expense. If you take the latest year's figure for all non-interest-earning expenses - such as advertising, personnel, and equipment - and divide it by the total loan portfolio, it should come out to about 1.5 or 1. 7 percent or less. ''A lot of institutions have 2.2, 2.5, or even higher,'' Gray notes. ''That is extremely inefficient.''

3. The number of real estate loan dollars per share. This is the mortgage portfolio divided by the number of shares being offered. The more loan dollars there are supporting each share, the better it is for the investor. Many of the large S&Ls in California that have been selling stock in the last year have more than $300 in real estate loans to cover each share of stock. Some have as much as $600 of loans per share.

Finally, if you are serious about buying S&L stock, it may be a good idea to get the opinion of more than one brokerage house. Brokers involved in underwriting - handling the sale of a stock issue - are not permitted to talk about estimates of what that stock might earn.

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