How to add ECB Menu Item for specific SharePoint List or Library
To add ECB Menu Item for specific SharePoint List or Library, first you need to know the Registration ID for that list. To get that, go to the desired List's Settings page and then select Advanced Settings. In that page, enable Management of Content Types and select OK. Now in the List settings page, go to the Content Type Section and select the Content Type listed there.
So in my case, when I selected Document content type, it took me to the next page and in the URL of the next page you have the 'ctype' listed at the end of the url:
Copy that CType as that is going to act as your Registration ID.
Next go to your Visual Studio solution and open Element.xml file. Change the RegistrationType's value to 'ContentType' and place the copied CType value to the 'RegistrationId's value field:
Now just save the file, and deploy the solution from visual studio if working in your development environment or else install and activate the feature to notice the custom ECB Menu only appearing in the specific list :)