Ah! Very nice...saw it in the catalogues earlier, but noticed the brazeons on the top side of the chainstay and thought it was an above the bracket routing.
Yeah this one may have had the stop removed from atop the chainstay and rebrazed underneath later? Just guessing. Mrkawasaki's seems to have a stop atop the chainstay but no tunnels brazed atop the bracket.
As mentioned, the Under BB cable routing puts it late late 70's or into the 80's.........does the rear dropout holding the mech have two little holes in it?