Some one appears to have three AT&T 3B2/300 manuals, cables diskettes, sadly
in Brazil and has posted some pictures in a Facebook group
https://www.facebook.com/groups/vintagecomputerswapmeet/permalink/3954289997
980016/
You can open the link in an incognito, sandboxed, VPNd session and still see
the post..
Dave
G4UGM