WHEN Postmaster General Anthony M. Frank went before the Senate Subcommittee on Federal Services, Post Office and Civil Service last month, he confirmed what most stamp collectors have known from the day that they opened their first stamp album.

The machines that cancel the nation's letters cannot distinguish between a 25-cent stamp and a 10-cent stamp, Frank acknowledged. The machines can detect if there is a stamp on the letter, but as Frank conceded, they can't read its value.

To the surprise of some senators, who suggested the Postal Service may need better machines, Frank said the Postal Service has a built-in detection system that screens virtually every letter. They are the nation's 240,000 letter carriers, he said.

This Friday, as a major stamp show opens in Arlington, the service will introduce its latest detection aid for letter carriers: bigger, brighter high-valued stamps.

Instead of the small purple stamps that carry the image of William Jennings Bryan and an equally small "$2" sign, the Postal Service is releasing a bold, multicolored commemorative-sized stamp featuring a yellow bobcat and a big, clear "$2."

The stamp is the first of three high-valued stamps that will feature colorful wildlife designs. They will replace the $1 and $5 stamps in the "Great Americans" series, which respectively carry portraits of Johns Hopkins, the Baltimore philanthropist, and Bret Harte, the western novelist.

Postal officials say the redesign was prompted by concerns that the values of some of the nation's so-called "regular" stamps, the ones regularly stocked by most post offices, were not easily detectable. Worried by the prospect of workers confusing a 2-cent stamp with a $2 stamp and faced with a deficit of about $1.5 billion, the Postal Service had reason enough for a major redesign.

The first stamp created to end the confusion was a new red 5-cent stamp released in February in Puerto Rico and honoring the island's first governor, Luis Munoz Marin. Instead of a large "5," it carries an "05" value, a typographical device designed to keep workers from confusing it with a $5 stamp. The zero is to be added to all new stamps valued at less than nine cents, officials have said.

There's no way that the new bobcat stamp can be mistaken for a cheaper stamp. Printed by the Bureau of Engraving and Printing through a combination of the offset and intaglio processes, the stamp resembles more a commemorative than a regular issue.

It was designed by Chuck Ripper of Huntington, W. Va., the wildlife artist who also created the service's widely acclaimed set of 50 wildlife stamps three years ago. Gary Chaconas and Dennis Brown of the bureau engraved the stamp.

What makes the bobcat stamp different from that series is that the new stamps don't identify the animal. Reflecting the service's desire to minimize lettering, the stamps contain only the required 25-cent value and the lettering "USA." Instead, the margins will carry the wording "American Wildlife, Bobcat (Lynx rufus)," another recent innovation to give postal clerks added information about the subjects of the stamps they sell.

The bobcat stamps will be released during 9:30 a.m. ceremonies at the Sheraton National Hotel, Columbia Pike and Arlington Boulevard in Arlington, before the opening of the Napex '90 stamp show. Gordon C. Morison, the assistant postmaster general, will speak at the stamp dedication.

The Napex show, which will feature 300 exhibits and 50 dealer booths, is open from 10 to 7 this Friday, Saturday 10 to 6 and Sunday 10 to 4. Admission is free.

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