http://www.americanradiohistory.com/ has pdfs of a very wide range of radio magazines. Take a quick look there are a large number of magazines on the site.
I have used this site over the years to look at older High Fidelity and Audio magazines but I noticed recently that the issues of Audio after 1970 are being added to the library. They have a good number of the October Buyer's Guides over the years 1970-1999. Other issues during those years are also becoming available.
Great resource for those that have this older gear and want to find some information.
These were the source of the only information I have on the Musical Fidelity AC-1 Moving Coil Phono Preamplifier I have. Got it in the mid-80s with a Magnepan Unitrac 1 tonearm and Dynavector 23R. Still have it as it does sound good. Now I know a little bit about it from those Annual Directories.