The Hardy Boys 67: The Outlaw's Silver by Franklin W. Dixon.

"The Hardy Boys 67: The Outlaw's Silver"
Armada Books
Language: English
Published in 1983, first Armada publishing.
Originally published by Wanderers Books in 1981.
In paperback format, contains the Hardy Boys story The Outlaw's Silver by Franklin W. Dixon in 190 pages, with black and white illustrations.
"The Outlaw's Silver is the title of a Hardy Boys Digest novel, written by Franklin W. Dixon. Sometimes it goes by the number of 67 in the series, a continuation of the original novels, and sometimes as the 9th, as it is the 9th published after Grosset & Dunlap lost rights to publish new Hardy Boys books in 1979.
The Hardy Boys are given clues which send them in search of the treasure hidden by the Outlaw of the Pine Barrens."