Sister Ann Moriarty, librarian at Holy Trinity School writes:

"I am a Roman Catholic sister and I live in a convent with 10 other sisters. I thought you might be interested in the enclosed piece of mail which was delivered here a few days ago."

The piece of mail to which she referred arrived in an envelope that brings to mind the recent growth of gimmick advertising. Prominent in this genre are the "special offers" that exploit True Believers -- the people who say to themselves, "There is a Santa Claus. There is, there is, there is, and he has singled me out for expecially good luck because I am specially deserving. One of thes days he's going to drop a bundle of loot into my lap, and today could very well be the day ."

The letter was from "Robert F. Little, Col. U.S. Army Retired." Its envelope was imprinted (in red and blue) with the legend: "A message of urgent importance to honorably discharge U.S. veterans about additional cash benefits .

* * * Please open immediately. * * * Attention Postmaster: Confidential material is enclosed. Please deliver at once." The back of the envelope was imprinted, "Personal documents enclosed ."

That was enough to tell me from the outset that the letter could be thrown away unread. But, dutiful cuss that I am, I went forward with my research .

I found that the letter was addressed to "Mr. Convent." The heading atop the page was, "Veterans Notification." Immediately below were the words: "As honorably discharged veteran you are automatically eligible for cash benefits, $50.00 A DAY, $1,500 A MONTH." A plastic "health benefits card" with raised letters and numbers that made it look like a credit card was pasted to the page. The card was made out to "Mr. Convent." The letter to which it was attached said, "Carry the card in your wallet at all times."

The letter was a mass mailing circular designer to sell hospital insurance to honorably discharged veterans of the armed forces. Interestingly enough, the back of the "health benfits card" carried the warning: "To all doctors, hospitals and whomever this may concern: This card is not evidence of insurance coverage."

Good heavens, Col. Litle, what made you think such a disclaimer was necessary? How could anybody old enough to be a veteran have been misled by your presentation? POSTSCRIPT

I, too, received an interesting offer yesterday. It was from (try not to laugh) the "Testing Department" of the "International Monetary Mint."

I was offered an opportunity to buy 10 "miniature Krugerrands" for $10 each plus a postage charge of $1 each. However, if I got my order in by Jan. 30 I could get an even bigger bargain: I could pay "only" $9 per coin and no shipping charges. The coins were described as being made of "solid 14-karat gold," with each coin containing .174 grams of gold.

With 24-karat gold quoted at around $555 a troy ounce, 14-karat gold would be worth $323.75 an ounce, or $10.41 per gram. A coin weighing .174 gram (174 thousandths of a gram) would contain $1.81 worth of gold. Even at the bargain rate of $9 per coin, the buyer of 10 coins would pay $90 for $18.10 worth of gold. At $10 a coin, the buyer would pay $100, plus $10 more for postage.

It is interesting to note that these coins are so light that all 10 can be mailed in a first-class letter for one 15-cent stamp. In fact, if there were such a thing as a weightless envelope, a single 15-cent stamp would be sufficient for 134 of these coins.

P.S.: I regret to report that I did not order any coins. The offer was extended not to me but to "N.E. Gold," whoever that is. I am as ineligible to cash in on this special gold coin bargain as Sister Ann is to claim Mr. Convent's veteran's benefits. METRO MAPS AVAILABLE

Two maps of Metro's bus and rail system are finally available. One shows Washington and suburban Virginia, the other shows Washington and suburban Maryland. Each map costs $1.

The maps will go on sale Monday at more than 500 locations. They can also be ordered by mail from: WMATA Office of Marketing, 600 5th St. NW, Washington, D.C. 20001. Bus time-tables are available free from the same office, or call 637-1261 to ask for the route you need.