Especially used of a firm, the colloquial American-English phrase to open the kimono, also to open one’s kimono, means to divulge information or secrets.

The following definition is from Braindump on the Blue Badge: A Guide to Microspeak, a glossary of the words and phrases commonly used at Microsoft, by Steven Greenhouse, published in The New York Times (New York City, New York) of Thursday 13^th August 1998:

Open the kimono A marvelous phrase of non-Microsoft origin, probably stemming from the rash of Japanese acquisitions of American enterprises in the 80’s, that has been adopted into the Microspeak marketing lexicon. Basically a somewhat sexist synonym for “open the books,” it means to reveal the inner workings of a project or company to a prospective new partner.

The phrase to open the kimono is based on the notion of opening one’s kimono and revealing one’s naked body. This notion occurs in The Goblin Fox and Badger and Other Witch Animals of Japan, by Ugo Alfonso Casal (1888-1964), published in Folklore Studies (Tokyo: Society of the Divine Word – Vol. 18, 1959):

Apparently it was believed of old that the wolf was shameful of sexual things, having no strong sexual instincts. He would never disclose his organ, but hide it behind his hanging tail. Should a person perchance see his sexual act, he or she would have to open the kimono and disclose his or her own organ, so as not to shame the wolf.

The earliest occurrences of the phrase to open the kimono, also to open one’s kimono, that I have found are as follows, in chronological order:

1-: From Strict Parent, Good Provider, the review by Stephen Carpender of The Sun Never Sets on IBM: The Culture and Folklore of IBM World Trade (New York: William Morrow & Co., Inc., 1975), by Nancy Foy—review published in The Evening Press (Binghamton, New York) of Thursday 27^th March 1975:

IBM is a successful company, always in the public eye. As one executive put it: “We have to decide how far we are going to open the kimono.”
“But this is one of the few companies that could afford to stand up under close scrutiny, to live in a glass house,” Mrs. Foy says.

2-: From Roughneck who bet on America and won, about Joseph E. Reid (1929-2020), President and CEO of Superior Oil Company, by Barbara Ettorre, published in the Daily News (New York City, New York) of Tuesday 23^rd June 1981:

“Up until two years ago, we weren’t that accessible,” said Edward T. Story, Superior’s vice president of finance recruited from Exxon. “Then, we started opening the kimono, so to speak. Joe is an astute delegator, who keeps an informal office. He’s an attracter of talent.”

3-: From Taxes, by Dean Foust, published in The Greenville News (Greenville, South Carolina) of Wednesday 18^th April 1984—Jerry Ingram was then a professor at the University of South Carolina:

“The Japanese equivalent to our chambers of commerce has made a policy that no Japanese firm will invest in a unitary tax state,” Ingram said.
The Japanese largely resent the tax because it requires them to open financial records for different worldwide operations, he said, which also allows competitors to take a peek. As industry analysts have said, the Japanese firms refer to this as “opening the kimono.”

4-: From Casino gambling company paying Meehan $100,000, by Rick Ratcliff, published in the Detroit Free Press (Detroit, Michigan) of Friday 12^th April 1985:

Patrick Meehan, the Detroit developer who has a plan to build 12 gambling casinos on Belle Isle, is being paid $100,000 to be a consultant for one year to Resorts International, a gambling hotel company based in Atlantic City.
[…]
“I want to be forthright,” he said. “That’s why I’m going to tell you this, to open my kimono. It’s an amount of money to cover office expenses.
“I pay two clerks. I pay a number of office expenses, an accountant and a bookkeeper. It just covers expenses really. It’s basically money to help the process proceed. It isn’t anything I view as a compromise of me or my ability.”

5-: From Word power challenge, a quiz published in Triad Business Weekly, a supplement to the Greensboro News & Record (Greensboro, North Carolina) of Monday 7^th April 1986—the phrase to open the kimono was defined as:

slang term for the revelation to prospective customers of company plans regarding future products.

6-: From Home Tours on Peninsula Raise Funds for Charity: Room With a View of How Other Half Lives, by Gerald Faris, published in the Los Angeles Times (Los Angeles, California) of Sunday 14^th December 1986:

Some of the wealthiest people in the South Bay […] freely throw their doors open to the masses in what has become a Palos Verdes Peninsula tradition: home tours that raise money—usually a lot of money—for community organizations ranging from church to arts groups.
The oldest, the Yule Parlor Parade of the Neighborhood Church in Palos Verdes Estates, took place for the 30^th year last weekend. […]
[…]
Real estate broker Robert Wardell, whose home was on the yule tour, said people offer their homes out of “pride of home, pride in the community and, hopefully, being able to contribute to the community itself.”
Wardell admitted that he was very reluctant to say yes at first because of the loss of privacy. “Gee, you open your kimono to everyone, baring everything, saying this is the way you live.”