Have you seen the book by John Odell "Secrets of Privy Digging"? He gives much practical advice about finding old privies, including some common areas for the old privies. You might be able to order through Interlibrary Loan at your local library or purchase it online. It is self-published, so you can find it on the author's website. (www.bottlebooks.com).
There are also many websites that provide advice to novice privy diggers. https://www.privydigger.com/links.htm (this is just one example....many others come up on a google search). You may have seen some of these.
This is not a subject we have much information on, unfortunately, aside from the book by John Odell. Some bottle collecting sites might have information for you as well. I am attaching a bibliography on bottles, in case you are interested.
