The Krantz Catalog 6b may be found as a PDF here:

http://www.mineralogy.eu/archive/Krantz/Krantz_6b.pdf

Here is another path to find it:

Go to "The Virtual Museum of the History of Mineralogy" at
http://www.mineralogy.eu/index.html

Then put the cursor on "Collections" and a box will open. Click on "Crystal Models".

This page will open
http://www.mineralogy.eu/models.html

On that page, click on "Krantz wooden crystal models archive".

This page will open http://www.mineralogy.eu/models/archive_Krantz.html

Here, scroll down to "Set of 416 crystal models arranged by P. von Groth".

Click there for the PDF of Catalog 6b.

Feel free to search that website for further Krantz catalogs and other interesting info about mineralogy.