THE 200TH anniversary of the Constitution and the 100th birthday of the nation's certified public accountants have very little in common. That is until you examine the designs for the commemorative stamps honoring them.

The stamps were designed and coordinated for the Postal Service by different teams of artists and designers, but have strikingly similar design elements. Each is a vertical stamp and each features a writing instrument, diagonally placed across the stamp, as its central element.

On the Constitution stamp it's a white quill pen against the yellowed background of the document and on the accountants' stamp it's a steel pen against a ledger book and an olive background.

"Happenstance and coincidence," says Jack Williams, an art director who worked on both stamps. Both Williams and Dickey Rustin, manager of stamp information for the Postal Service, say the stamps were designed separately and the artists, one in McLean, the other in New York, never saw each other's work.

The CPA stamp, by Lou Nolan, a Northern Virginia graphic artist, took more than 18 months; the designers discarded such ideas as green eyeshades, which accountants felt were trite and stereotyped their profession.

The stamp will be issued in New York September 21 to honor 100th anniversary of the American Institute of Certified Public Accountants. Nolan's previous designs include the 17-cent dogsled stamp and the 3.4-cent school bus stamp, both part of the transportation stamp series.

Howard Koslow of East Norwick, New York, designed the stamp to be issued September 17 in Philadelphia marking the signing of the Constitution. Howard Paine served as art designer for Koslow's 22-center, the latest of four commemoratives by Koslow (whose earlier stamps were the Brooklyn Bridge, Tennessee Valley Authority and rural electrification issues).

Diagonal designs might seem to be common elements in verticallydesigned commemoratives, but a quick review of the issues of the past two decades shows surprisingly few. Some of the stamps issued for the 1984 Olympics did make use of strong diagonals, but most verticals have been just that: stamps with strong vertical elements.

The Citizens' Stamp Advisory Committee, which advises the Postmaster General on the appropriateness and design of new stamps, did not initially approve of the CPA stamp, which came directly from the PMG's office.

The committee has existed since the Eisenhower administration; some stamp collectors say it doesn't matter, but others say the 15-member panel can make a difference.

PMG Preston R. Tisch has moved to give graphic artists greater influence on the panel, which meets in secret, by naming to it Richard Coyne of Palo Alto, California, founder and publisher of Communication Arts, a graphic arts journal; Steven Heller, art director of the New York Times Book Review; and Beatrice Sanchez of Bloomfield Hills, Michigan, dean of the Cranbrook Academy of Art.

They will fill positions created by the deaths of Clinton T. Andrews Jr. of Anchorage, a flower-shop operator, and Edward A. Beard, a retired D.C. Superior Court judge; and the resignation of novelist James Michener, probably the best-known member. Michener, who recently underwent heart surgery, cited press of commitments. The recent death of Wilbur J. Cohen of Texas, the former health secretary, creates another opening.

Collectors seeking a first-day cancellation of the Constitution stamp have until October 17 to send envelopes with stamps affixed to: Customer-Affixed Envelopes, Signing of the Constitution Stamps, Philadelphia, PA 19104-9991. For 22 cents the postal service will affix the stamps if you request them from: Signing of the Constitution Stamps, Postmaster, Philadelphia, PA 19104-9992. Details on the CPA stamp will be anounced later.

Bill McAllister is a member of The Post's national staff.