Two weeks ago producer Dino De Laurentiis denied that he was selling shares in his $40 million production of "Dune." But last week a confidential financial prospectus was revealed, offering just that to a small number of qualified investors: 65 partnership units in the big-budget science fiction movie directed by David Lynch ("Eraserhead," "The Elephant Man").

If the $24,500 units sell well, it may ease the financial burden on De Laurentiis, whose film is considered one of the chanciest of the Christmas season. Exhibitors saw it for the first time last week: Some loved it and foresaw big business, but many others quietly grumbled that it was something the young science fiction audience may not stand for -- boring. Among Lynch's more vocal supporters, however, is "Dune" author Frank Herbert.

Another financial prospectus that recently came to light provided an illustrative picture of just how Tri-Star hopes to become an instant major studio. Tri-Star produced 17 films that have been or will be released this year, and has scheduled another 18 for 1985. The studio has also obtained outside financial guarantees that will cover all the production and acquisition costs for every one of those films.

Because Tri-Star is a joint venture of Columbia Pictures, CBS and Home Box Office, it was able to use Columbia's existing distribution system and to make lucrative exclusivity deals with HBO and CBS. HBO, in fact, is the largest single contributor to the Tri-Star kitty: It paid $67 million for its initial partnership, and has now guaranteed an additional $50 million through 1985.

The financial details came out when Tri-Star drew up a stock-offering prospectus. If the government gives it the go-ahead, the studio will be looking to raise another $75 million or so that way.