Miller Barber is nicknamed "Mr. X" because, after 21 years of consistent performance on the professional golf tour, he remains a mystery man to the public and his colleagues alike.

The rotund 48-year-old Texan with the double chin and homely swing has managed to win 11 tournaments and $1,374,591 in official tour earnings, placing him ninth on the all-time list-all without ever making any serious inroads on real celebrity. He is truly "the unknown millionaire."

Barber-familiar but eminently, almost compulsively forgettable in his ever present sunglasses and Amana cap-had made a career out of being colorless and expressionless.

Usually, his most flamboyant gesture is to take off his golf glove. If he gets really exited, he might stand on one foot for two seconds and think about smiling.

How appropriate, then, that Mr. X should equal the course record of 64 at the Augusta National Golf Club and yet cast hardly a ripple of excitement or interest over the 43rd Master Tournament.

If he could have counted only the best 18 holes he played today, Barber would be in second place after three rounds, instead of tied with four other players for sixth place, seven strokes behind leader Ed Sneed.

But he had to play 21, and thd last two got him: a careless two-foot putton 17, and a drive that faded into a tree trunk and set up a second shot into a sand trap on 18, bogey, bogey, to finish a day that had begun so promisingly.

Mr. X, Would you sign out please?

Barber was one of a dozen players still out on the squishy-soft course when darkness fell Friday evening, forestalling completion of a round that had been interrupted for two hours by afternoon thundershowers.

His last shot in the gloaming had been to chip in from 85 feet for an eagle at the par-5, 520-yard 15th. Fittingly for a player who has made an indentity out of not having one, hardly anyone witnessed the feat.

"I kept my sunglasses on right 'til the bitter and, even though it was so dark I could hardly see," Barber reported. He thinks the spectacles are an essential part of his image, and he is right. With them on, he can travel as himself and still go incognito.

Barber was back out on the course at 8:30 this morning, when the sun was still low in a faultless blue sky that foretold of a sublime spring day to com. Large galleries. including a number of stunning young things and barefoot contessas, were already streaming through the entrances-but not to watch this ex-marine of ample girth and slope-shouldered gait.

Barber likely would not be able to get dinner in town without his American Express card, and so why should there be a mass procession to see him? He parred the 190-yard 16th; birdied the 400-yeard 17th with a drive, an eight-iron approach shot to 12 feet and a fine putt; and parred the 420-yard 18th with a three-wood, six-iron and two putts from 40 feet out on the fringes, all in relative privacy.

"I didn't have a makeable putt at 18, but I was trying to make it, anyway," drawled Barber. He had 11 threes in all and came within 18 inches of breaking the course record he now shares with Lloyd Mangrum, Maurice Bembridge, Jack Nicklaus, Hale Irwin, and Gary Player, who shot 64 on the final round to win the Masters green jacket last year.

Mr. X, who at 5-11 and 210 pounds does not have quite the physique for the beltless slacks he prefers, teed off for his third round at 1:44. By now, the day was in its full, radiant glory, and for nine holes, so was Barber.

Barber has a peculiar stance, a whippy takeaway, and the most grotesque of flying elbows. If you saw the means without the end, you would surely throw this unorthodox swing in one of Augusta's plentiful litter receptacles. But Mr. X's hands arrive throught the hitting area just fine, and he was crushing the ball today. He birdied the par-3 sixth and par-5 eighth to go seven under for the tournament at the turn.

"He's hitting the ball a ton," said one galleryite, not meaning to imply that Mr. X was carrying his weight.

Barber bogeyed the par-4 10th, rolled in a 25-foot downhill putt from the fringe to save par from a trap on the par-3 12th, and got around "Amen Corner" one under with a birdie at 13. Another mighty drive left him only a two-iron shot to the green on this spendidly watery, 485-yead par-5, and he very nearly allowed himself a smile when he two-putted for the bird.

But then it started to come apart.

"I like the image of Mr. X. It gives me an identity. It separates me from the masses. There is Buffalo Bill (Casper), The Golden Bear (Jack Nicklaus), and Mr. X,''Barber had allowed Friday evening. Coming from him, this sounded like a whole Johnny Carson monologue.

Had Miller Barber, the leader of the land, come out of the closet and become a personality?

Hardly.

His disheartening finish today left him in no mood to be jocular, or even to talk, although he now has a unique place in golf history. Surely, he is the only player ever to set the Masters couse record, and finish bogeybogey the same day.

And no one cared.