Writing Credits for David Holzman's Diary

David Holzman's Diary (1967), an early example of the mockumentary genre, has no onscreen writing credits (presumably to keep up the illusion that it is a documentary). Every contemporary source I could find, including IMDb, AFI, and the Library of Congress National Film Registry personnel credits, either leaves out the writing credits or credits Jim McBride (the film's director) as the writer.

Oddly, a 1970 paperback edition of the screenplay contradicts this. Its credits read: "A screenplay by L. M. Kit Carson from a film by Jim McBride".

ISBN: 9780374135201

Carson, the film's lead actor, has 11 writing credits on IMDb from 1971-2003. McBride has 9 credits (including two uncredited) from 1967-2009.

While the contemporary sources are pretty consistent in naming McBride as the "writer-director", I uncovered no primary sources corroborating that. Instead, I found a book released much closer to the film's release date crediting Carson as the author.

What could be the reason for this discrepancy? Do the credits of the paperback screenplay have any bearing on how the film itself should be credited?