I have an paginated category URL that displays all items in a category.
Each item has its own individual view page.
In the event that an item has gone permanently offline I have implemented a 301 redirect to the first page of the category. I have added a url/GET parameter to indicate a small offline content flash message should be displayed at the top of this category page.
which has a canonical link in its head:
with an additional message:
'Sorry, this item is no longer available. Please check out our similar items'
I have added this message parameter to the URL parameters in my webmaster tools, suggesting that it doesn't change the content.
Is this correct? It technically does change the content a minimal amount.