FOR EVERY stamp collector who thinks the stamp is the most important thing on an envelope, there is undoubtedly an equal number of letter writers who would give paramount attention to the postmark.

Come Valentine's Day, millions of postmark advocates will be dashing to often small and obscure places -- Loveville, Md.; Lovejoy, Ill.; Valentines, Va.; Kissimmee, Fla. -- to get their valentines postmarked with an added message.

Next month, as the Postal Service introduces its 12th Love stamp, there is an opportunity to get both a special Valentine's cancellation and a new stamp all in time for a Valentine's Day greeting.

The new stamp goes on sale Feb. 6 in Loveland, Colo., a Denver suburb which has made a booming business out of canceling Valentine's Day greetings. Since 1947, the Chamber of Commerce there has been seeking a slice of the Feb. 14 business, offering envelopes imprinted with a valentine design, or "cachet."

According to postal officials, it's been a growth business. In 1947 the Loveland post office received about 300 valentines for remailing. Last year postal officials estimate the number was 300,000.

If the design of a red heart emerging from a blue envelope proves as popular as recent first-day cancellations, Loveland should easily pass 1 million this year. The first-day ceremony is being held there eight days before Valentine's Day to ensure that most post offices have adequate supplies of the 900 million Love stamps before most valentines go in the mail. Letters sent to Loveland in advance should easily reach their destinations by Feb. 14, according to postal spokesmen.

Uldis Purins of Newton, Mass., designed the new stamp using a big red heart, "the universal symbol" for valentines, as the central element. It's the third year the Postal Service has used a heart theme in the popular stamp. Last year, it offered a heart-shaped world and the year before that a pair of bluebirds between a red heart.

Purins, a graphic arts designer, said he was happy with his design because "it's nice and simple like a stamp should be." He completed the design, his third stamp, last summer and it quickly won approval of postal officals.

Like other stamps in the series, it carries only the words "Love" across the top of the stamp and the required "USA 29" on the bottom.

American Bank Note Co. printed the stamp on gravure presses. It will be sold in sheets of 50 stamps. In addition to valentines, the stamps is expected to grace many wedding announcements this year.

TWO CHANGES in the nation's money have been announced by the Treasury Department agencies that make coins and currency.

In an effort to make its gold Eagle coins "more appealing" to consumers, the Mint has disclosed that it will begin placing the date on the coins in Arabic numerals (i.e. "1992") instead of the Roman numeral year dates that U.S. gold coins long have carried. A Mint statement said the change "is designed to make the Eagles easier to read and more attractive for gift-giving that is often associated with birthdays, weddings and graduations."

It also said the change will not have an impact on the price or profits of the coins.

The other monetary change was widely expected. The Bureau of Engraving and Printing announced it will soon be releasing the first $50 bills that contain the same security devices added to the $100 bill last year.

The bills will carry a polyester security thread through which light can pass, microprinting of the words "United States of America" around the portraits on the bills and other unspecified security devices. The actions, which have been in planning for almost a decade, are to be added to all other bills except the $1 by 1995.

Treasury officials have said the steps are designed to thwart counterfeiters and use of high-resolution color copiers to duplicate the bills. Copiers should be unable to duplicate the opaque slit in the bill that the polyester threads produce when the new bills are held up to a light.

INDIVIDUALS wishing first-day cancellations of the new 29-cent Love stamp may purchase the stamp at most post offices after Feb. 7. Envelopes bearing those stamps should be mailed to: Customer Affixed Envelopes, Love Stamp, Postmaster, P.O. Box 9998, Loveland, CO 80538-9991. Postal workers will affix stamps for 29 cents each on up to 50 addressed envelopes at: Love Stamp, Postmaster, P.O. Box 9998, Loveland, CO 80538-9992. All requests must be postmarked by March 7.

Bill McAllister is a member of The Washington Post national staff.