Well, checking these titles on bookfinder, I come up with an
ordinary Detective Book Club edition as well, which leads me
to assume that 1969 (or 1970) *is* the date of publication.
I'm not sure why you think that. If a book carries a copyright but no
publication date then, unless there is other evidence to pin down the
date of publication, all one can assume is that it was published on or
after the copyright date.
my impression was that the titles were always contemporary
mysteries, and I'm wondering why, after 28--or even 35--years,
they should do this volume.
That Zane Grey website has a lot of info on Walter Black. Apparently
they did a lot of reprints of Zane Grey material during the 1960s:
The Walter J. Black Editions were reprints of seventy-three of
Zane Grey's titles with one additional title appearing as a First
Edition. The series also included a biography of Zane Grey.
Produced in the 1960's, they were bound in red and tan cloth and
originally came in a rice paper cover.
It goes on to discuss the value of these editions (most of them are of
little value, but there are one or two exceptions).
So Walter Black were reprinting Zane Grey stuff in the 1960s and several
of these reprints (according to http://www.zgws.org/zgfechrono.html
were limited "leatherette" editions. It's not unreasonable to suppose
that they were reprinting other material from decades earlier and that
some of that, too, came out in limited editions with a leather binding.