It seems the "Selling Gold?" plugin only accounts for admin-side things, and does nothing with catalog-side (storefront-side) files.

You'll probably also need to duplicate these:

/includes/modules/pages/product_info/*.*
as
/includes/modules/pages/product_silver_info/*.*

and
/includes/templates/template_default/templates/tpl_product_info_display.php
as
/includes/templates/NAME_OF_YOUR_TEMPLATE_FOLDER/templates/tpl_product_silver_info_display.php


It's also worth noting that the "Selling Gold" plugin is based on files from ZC v1.3.8 ... which is from a very long time ago. So the files it's having you replace probably shouldn't be replaced directly. Instead you should "merge" their changes into your own files. That's done by comparing the files they supply against the files from original v138 (using a tool like WinMerge) to identify the customizations done, and then copy/merge/adapt those customizations into your new site's files. And of course then test.