Yeah... it's a pity (an understandable one in way) that I couldn't find more older more technical brewing books scanned online for quick reference. But at least some resources quote them. There's definitely a fair bit of older brewing books sitting in dusty corners of certain libraries that would be interesting to read. :)
This modern printing of a book from 1785 (https://www.amazon.com/Hydrometrical-observations-experiment...) for instance would've been interesting to view to get a perspective from a century earlier (at least for British porter). Unfortunately I can't find a digitized version online, but querying on that book led to this link, (https://books.google.com/books?id=GSTnBAAAQBAJ&pg=PT24&lpg=P...) does quote a 1784 porter at OG 1.071 FG 1.018 (about 7%). The full reference would probably be more helpful to determine a better picture of the historical range back then.
I believe the hydrometer only came into use in brewing science in the mid to late 1700s; before then, data definitely would've been a lot sketchier.