It was an honest mistake. The upstate New York wallet manufacturer simply wanted to show how nicely a Social Security card slid into the custom slot. The year was 1938 and the number belonged to the entrepreneur's secretary.

But thousands of wallet buyers -- 40,000 at the peak -- who received the sample card took the number as their own, forcing Social Security Administration officials to void the number. In 1979, four citizens are said still to be using it.

In a society increasingly dependent on the transistorized nod of a computer to prove citizen identities, the wallet episode highlights the attitude many Americans have about Social Security numbers. It isn't just another number -- it is The Number.

A citizen who tries to keep The Number confidential soon encounters a host of difficulties. Bank officers hand back "incomplete" application forms, and grocery chains refuse to grant check-cashing privileges.

"We don't encourage the use of Social Security numbers for general identification purposes," says Dr. Abe Bortz, historian for the Social Security Administration, "but it is gradually becoming -- despite our efforts -- the universal identifier.

"After all, numbers are better than names, because you can have a million John Smiths born on such and such a date, but a nine-digit number will always be unique."

The superiority of digital identities may be debatable, but the consensus among banks, schools, insurance companies, government agencies and many other institutions is that the nine-digit code says it all.

As originally conceived, the Social Security number was only to be used to track Social Security tax payments during a worker's career, and to record benefits when the need arose. Accordingly, employers asked for the number as a bookkeeping procedure.

Banks began asking for The Number about 10 years ago, when the Internal Revenue Service announced that interest payments should be reported directly to the IRS. Today, virtually every type of financial institution asks for The Number on its application forms.

Richard Hess, a Social Security claims specialist, says "the use of Social Security numbers by banks and the IRS is something we have no say in. That's something those organizations go after on their own."

The convenience of a unique number, one that is recognized by the federal government, has attracted new fans in recent years.

Donald Cragan, public relations director for the Massachusetts Registry of Motor Vehicles, says The Number is used on drivers' licenses for the "obvious reason -- everybody has one." License applicants in Massachusetts can choose to withhold their Social Security number but must fill out another form in order to get a number assigned.

"About half of the states try to use Social Security numbers on auto license applications, and of those, half are optional," Cragan says. "It seems to be a simpler way out."

Colleges and universities and some public and private secondary schools frequently ask students to supply The Number. Applications for financial aid require it because they usually involve on-campus employment.

Douglas Hartnagel, dean of admissions at the University of Massachusetts, says, "We ask for the student's Social Security number, but it's optional. It's just another way of cross-referencing our student identification files, like birth dates."

"There is resistance to the use of Social Security numbers as identification -- in small ways," says Mindy Lubber, legislative coordinator for the Massachusetts Public Interest Research Group. "There's a movement to prohibit the use of The Number as the key to new electronic banking systems."

Art Erocolini, assistant editor of Consumer Tribune, says the "universal appeal" of the "universal number" could lead to violations of personal privacy. He explains:

"Once you have someone's Social Security number, I wouldn't doubt you could pick up everything there is to know about a person -- tax records, criminal convictions, anything."