Joe, thank you for your reply and excuse me for the long delay in answering but I manage just the WordPress part of a site and I had to ask the site manager for the info.
As fa as I can understand all plugins are in the same situation including the plugins folders:
public_html: read, write exec for owner permission, read, exec for group permission, none for public permission
wp-content: same as above but read for public permission
plugins: same as wp-content
wp-to-twitter: read, write exec for owner permission, read, exec for group permission, read, exec for public permission
None of the above folders have the "subfolders included" marked.
I checked other different working plugins folders and they share the same definitions.
Thank you very much for your help, Massimo
P.S. I have the "images" of the above analysis and, if required, I can send them to you via email.