Nonfiction briefly noted. . .; Shootin' holes in westerns; God Bless You, Buffalo Bill, by Wayne Michael Sarf. Rutherford, N.J.: Fairleigh Dickenson University Press (distributed by Associated University Presses, 4 Cornwall Drive, East Brunswick, N.J. 08816). 279 pp. $27.50.

Remember the movie ''The Comancheros,'' in which John Wayne defends the Good Guys with a hail of bullets from his Colt .45 ''Peacemaker''? The Duke's shootin' was even fancier than it looked - because the plot was set before 1845, whereas the Peacemaker wasn't invented until after the Civil War.

A small mistake. But these things add up. As Old West aficionado Wayne Michael Sarf observes: ''The same easily avoidable errors recur in film after film, ultimately impressing themselves on the public ... as bits of bona fide Western Americana.''

With good-natured irony, Sarf points up assorted anachronisms, distortions, and downright falsehoods from the earliest westerns to the present. He explains , for instance, that the low-slung holster - ubiquitous on the screen - didn't come into use out West until about 1900. And he describes how Hollywood has glorified dubious heroes such as Wild Bill Hickok, while ignoring better-qualified lawmen with less colorful names. (Ever hear of plain old Bill Tilghman?)

In a sense, the westerns make too easy a target; most of them didn't even try to get their history straight. But there's more than enough infomation here to keep you awake during commercials on the Late Show.