Or use the Internet Archive / WayBackMachine (from 5.1.): https://web.archive.org/web/20180105140154/http://www.unixpr...
There's no "cache://" "protocol". This is simply Google's search query parser being extremely forgiving with "cache://" vs "cache:", which is the correct syntax (as documented on various sites on the internet).
When you type "cache:" or "cache://" into Chrome (FF not tested) you'll see the top result it's about to execute is a search. It just sends the string to google.com.
If you go to google.com manually and input "cache:www.example.com" as the search query, you won't get a results page, you'll go directly to the cached page.
Edit: After about 5 minutes curl answered: Last-Modified: Mon, 27 Dec 2010 20:40:54 GMT (which is unsurprising given the web.archive.org info showing amends of the page)