Gil Underwood looked at the envelope in which his Pepco bill had arrived and noticed that it bore only 12 cents in postage. He was understandably curious.

"Why do I pay 13 cents for first class postage," he wondered, "when Pepco pays only 12 cents?"

There is a good answer to Gil's question. To trace it to its origin, we must go back more than four years to Jan. 18, 1973. On that date, the Postal Rate Commission opened public hearings, during the course of which the United States Postal service and its patrons discussed ways in which they might cooperate to achieve mutually beneficial economies.

The hearings ran for more than three years and resulted in a plan for a "presorting" program that went into effect precisely one year ago, on July 6, 1976.

The new program offered a discount of one penny per letter to mailers who are willing to meet a detailed list of USPS conditions. A penny per letter isn't much to the average citizen, but it amounts to substantial sums for public utilities, department stores and other firms that mail out hundreds of thousands of bills every month.

To qualify for the 1-cent saving, mailers must meet many requirements, including these: mail more than 500 letters at a time, accompany each mailing with the proper USPS forms, pay an annual fee of $30, deliver the cut-rate mail to "a specified postal facility within a time period designated by the postmaster," ZIP code all mail, imprint on each letter or card (below or immediately to the left of the stamp or postage meter mark) the legend "Presorted First Class," and - most important of all - presort all letters and cards according to the ZIP codes to which they are addressed.

It costs the mailer something to do this work, but the mailer's work saves money for USPS. The penny "discount" is considered a good estimate of what the presorting is worth.

"We believe," says USPS, "that mail presorted and delivered to us in this manner results in savings to mailers and to the Postal Service, as well as - ultimately - consumers."

The presorting program got off to a modest start last summer, but by October had won good acceptance among large mailers. Since October of 1976, USPS has tallied 1.2 billion pieces of mail handled under the cut-rate plan.

One reason the public has so little awareness of the presorting program is that the mandatory imprint "Presorted First Class" has not yet been incorporated into the postage meter plates used by most business firms. It is still an additional imprint that often doesn't register too legible. On the Pepco envelope that Gil sent me, for example, the "Presorted" legend is so indistinct that one is unlikely to notice it unless he knows where to look for it.

Another reason the public knows so little about the presorting program is that the average citizen will never have occasion to use it, and the news media therefore gave scant publicity to the Postal Rate Commission's three years of hearings or its subsequent decision to put the program into effect.

The entire matter is a good example of the tremendous amount of official activity that takes place in Washington with little or no mention in the news media. Those of us who can't take the time to read every word in the Congressional Record and the Federal Register must of necessity remain unaware of much of the routine work that is done here.

This is also true of official actions taken at state, county and municipal levels. Until we see an $8 "system charge" on our gas bills, or a letter from Pepco with 12 cents worth of postage on it, we don't realize what manner of grist has been moving through the governmental mills.