Before making the printing of 1866 issues (Scott 14), the Post Office Printing Bureau was made a deep cleaning of the plates, as in the previous years has accumulated much rust and dirt.
To check the quality of the cleaning was made some impressions on unwatermarked paper.
A few years later appeared on the philatelic market these test prints, and are those that are know as "reprints"

These reprints were never postaly used, therefore don't have postmarks.
The paper of these stamps was made by hand and in some stamps watermark is more visible than in others...but the "5" should be there. Test/try with benzine.
Watermark "5" can be found in any of the following positions :

The best catalog on Chilean stamps is the specialized edited by the Philatelic Society of Chile.