It had to have happened this way: Polyester producer strides into network executive's office one day slapping a semiread paperback book and exclaiming, "This 'Black Beauty' thing is surefire. It can't miss.

"All we have to do is change everything about it but the horse."

"And so it came to pass - like a kidney stone: NBC's five-part "Black Beauty" premiering tonight at 8 on Channel 4 and continuing in one-hour installments through Saturday. The only interesting question this laborious and charmless adaption raises is whether children exposed to it will find their appetites whetted to read the book or just be further conditioned to accept humdrum TV shows as substitutes for entertainment.

Peter S. Fischer, who adapted Anna Sewell's long-popular book, obviously sees it not as a tract on behalf of gentleness toward creatures great and small, but as a pretext for trotting out a pact of neurotic adults who suffer and squabble and cough while a disinterested horse watches from the corner.

Neither Fischer nor director Danniel Haller do much to keep the story flowing or make the characters believable. Naturally, the first-mammal narrative of the book has been jettisoned and the horse's autobiographical commentary replaced with corny bits spoken on the soundtrack by David Wayne.

Fischer has a beck of a time getting the poor old horse into the story. When a young man is killed in a riding accident, his mother grieves for a year, then rides Beauty out to the gravesite for some heavy weeping. She is miraculously cured of her grief when Beauty saunters over and socks her in the head with his nose. Somehow this doesn't seem the sort of romatic adventure author Sewell had in mind.

The setting has been changed from the Tally-Ho territory of England to Maryland in the late 1800s. Nevertheless, the actors and director decided to stage a battle royal of dueling accents (British Isles variety,) with the funniest combatant clearly the over-bearing William Devane as a maniacally Irish groom.

"Didje hear that, Beauty?" Devane asks the horse. "Mar mood." He means "More mud."

NBC boasts "107 actors and 800 extras" involved in this production. The actors include such inescapable TV perennials as Martin Milner, Cameron Mitchell and Clu Gulager. The Kentucky location lends some sort of visual refreshment but the director really doesn't exploit the scenery, being more concerned with the spasms of tubercular old ladies, trumped up calamities and a general tone of unrequited groom.

One might have hoped at least for a kind of bucolic "little horse on the prairie" approach. But "Black Beauty" lacks even an acceptable bland TV mediocrity. You can not only lead a horse to water, it turns out, but you can also make him sink. 'Fifty Years'

It's time again for another edition of "Thank you, Orson," that occasional series of special specials in which a number of stars introduce slips from old NBC TV shows after first thanking Orson Welles who has in turn introduced them.

The fact that Orson and the stars in question were taped separately, and may never have come within 10 feet of one another in only a tiny hypocrisy when considered against the larger sham of "NBC: The First 50 Years - A Closer Look, Part Two," at 9 tonight on Channel 4.

Dressed in attire suitable for Oscar Wilde's funeral, Welles presides again over a collection of excerpts that seem meticulously selected to be as maddeningly brief, quixotically chosen, incompetently organized and generally misrepresentative as possible.

Since the two-hour special is devoted to dramatic shows, producer Greg Garrison wasn't able to squeeze in any tributes to programs he did with Dean Martin or Marty Feldman. This marks a first for "The First 50 Years." But the program is careful to contend that the quality and quantity of dramatic programming on television are as high as they ever were, which takes the prize as the big-bilge notion of the week.

And yet, some of the backward glimpses are fascinating: Judy Holliday and Tony Randall clowning in a 1954 comedy; Loretta Young telling a blinded patient that she can't return his love because she's a nun; Lunt and Fontanne in "Magnificent Yankee"; and a truly stirring scene from "The Sacco-Vanzetti Story" with Martin Balsam and Steven Hill.

Stars like Robert Redford, Mary Tyler Moore and Warren Beatty pop up in small roles they played before they became famous. And, a 1965, pre-windbag version of Orson Welles demonstrates how receptive the TV camera can be to an inspired virtuoso ham as he transforms himself into a purely and hokily magical Falstaff.

For all the clumsiness of these retrospectives, NBC has provided an inadvertent service or two. For one thing CBS now has a living blueprint on what not to do in preparing its own 50th anniversary celebration for broadcast next month.

The shows also have illustrated the carelessness with which networks have preserved their own best and worst material. A clip from a 1954 TV version of "The Petrified Forest" starring Henry Fonda and, in his only TV dramatic role. Humphrey Bogart, is a fascinating momento except for one little thing - some klutz lost the soundtract. The footage is mute.

Much of television's early history has already been lost through negligence or mishap. William S. Paley of CBS founded the Museum of Broadcasting in New York to house and display some of this material, but the museum is more of an active library than an official archives.

Thoset who cannot remember the past, especially in the most recorded century in history, are, like NBC with its fuzzy "closer looks," prone to betray it.