Would you like to know who's having a really merry Christmas this year? The investors who did their Christmas shopping last summer and bought stock in one of a dozen banks that do business in the Baltimore-Washington-Richmond corridor -- that's who.

Now that Christmas is here, those wise shoppers can sit back and admire the 10 to 25 percent gains they have made on their stocks just since Labor Day.

Talk about holiday cheer. What a warm feeling to sit before the fire and contemplate how that share of Mercantile Bankshares Corp. of Maryland grew from $37.50 on Sept. 4 to $44 on Friday.

Mercantile was only one of the several area bank stocks that climbed sharply. They include Suburban Bancorp of Maryland, Central Fidelity Banks Inc. and Dominion Bankshares Inc. of Virginia, Maryland National Corp., First Maryland Bancorp, Union Trust Bancorp of Maryland and Bank of Virginia Co.

The increases were part of a general upswing in regional bank stocks that brought prices to the highest levels in five years.

The credit for most of these gains goes to interest rates, which have been falling sharply in recent months. When interest rates fall, banks pay less to borrow money and they benefit from a wider spread between what they pay for funds and what they charge their customers. As spreads widen, and as a bank's asset base grows, bank profits tend to rise. Higher profits often mean higher prices for a bank's stock.

Among those watching the bank stocks rise -- with special satisfaction -- were Johnston, Lemon Research Director Charles T. Akre Jr. and bank analyst Gilbert W. Keech Jr. In a September report on 16 banks in this region, they targeted Suburban, Mercantile and Sovran Financial Corp. of Virginia as three stocks especially worth buying. All three were praised for their earnings growth and for their conservative banking practices.

Akre and Keech's timing was good. Since then, Mercantile has risen 17 percent. Suburban, recommended at $36.75, rose to $43.125, up 17 percent. Sovran, doing less well, was recommended at $34.25, and went up to $38.50, a 12 percent boost.

Big gainers included Maryland National, which rose from $37.50 to $44, an increase of 17 percent. First Maryland was up from $25.875 to $32.25, a 25 percent rise. Central Fidelity Banks Inc. of Virginia, was up from $24.375 to $28.25, or 16 percent.

More modest gains -- below the 15 percent level -- were registered by United Virginia Bankshares, First Virginia Banks Inc., Citizens Bancorp of Maryland, Riggs National Corp., Equitable Bancorporation of Maryland and Washington Bancorporation (National Bank of Washington).

Only American Security Corp., which had to increase its loan loss reserves, faltered, falling back to the $17 level of September.

The rising level of bank stocks posed this question for investors: Are the stocks, selling well above their book values, still good buys at their present prices or have they moved up so far that the window for potential gains is all but closed?

The answer, Akre and Keech said, depends on many things, including the direction of interest rates, the business climate and the management of individual banks. The stocks are clearly at the high end of their price range, the analysts said, but in an era of continuing low inflation and bank deregulation, the stocks might continue to appreciate, if somewhat more slowly. On the other hand, if interest rates were to start rising, the cycle might turn the other way and bank prices could start to slide.

One factor, sometimes forgotten, is the role played by institutional investors who buy large blocks of stock for various kinds of funds. One estimate is that 80 percent of the regional bank stocks were bought in recent months by large buyers, only 20 percent by small investors. Because fund managers tend to take their profits and leave, large selling could affect present stock prices.

While the analysts, who can never rest on yesterday's forecasts, try to figure out where these stocks go next, investors will be counting their profits all the way to the bank.

A couple of weeks ago, Scope Inc. of Reston announced it would buy 243,080 shares of its stock from the investment banking firm of Rooney, Pace Inc. of New York at a price of $5.50 a share. Rooney Pace acquired the stock in early 1983 as the result of a company restructuring in which two subsidiaries were spun off to stockholders.

Scope said it was willing to pay Rooney Pace a $1.50 premium over the $4 price. But then the stock dropped to $3.50 a share and Scope's financial advisers objected. The price gap was too wide, the advisers said, and might invite shareholder lawsuits. So, although the company could have raised the $1 million needed for the purchase, said President Ed Driese, the plan was dropped. Scope handles defense work, including electronic warfare projects.

Radiation Systems Inc. of Sterling, Va., a firm that manufactures antennas and antenna systems, has made "significant progress" in sales and earnings since the fall and its prospects are growing brighter, say the analysts of Scott & Stringfellow Inc. of Richmond. The analysts forecast $1.25 a share earnings for 1985 and $1.60 for 1986, compared with $1 for 1984.

On Oct. 30, Radiation Systems booked its largest project ever -- a $20 million, five-year contract with Associated Universities Inc., a nonprofit corporation that operates the National Astronomy Observatory under a contract with the National Science Foundation. Radiation Systems stock, which was as high as $27.75 in 1983 and as low as $9.25 in July, is now at $13 a share.

On Dynalectron, a McLean, Va. firm, Scott & Stringfellow has changed its recommendation from a "long-term purchase" to a "hold." Research Director Michael L. Mead has cut his earnings estimate for 1984 from $1.30 to 95 cents a share and, for 1985, he has dropped the estimate from $1.90 to $1.50 a share.

"Some elements of the specialty contracting area continue to be weak -- synfuels, industrial insulation and heavy construction -- while commercial aviation services and government services were strong," Mead wrote. Dynalectron is in the midst of a program to buy back 1 million of its shares, and as of Nov. 5 had acquired 640,657 of those shares.

Hazelton Laboratories Corp. of Vienna will buy shares of its stock on the open market from time to time for use by its employe stock purchase plan. The company has reserved 250,000 shares for the plan. Up to now, 102,000 shares have been purchased by plan participants.