The early 80s 6nn series frames had Reynolds 531 main triangles brazed in Waterloo, which were mated with rear triangle subassemblies and forks brazed in Japan. The tubing used for the forks and subassemblies varied (Ishiwata, Tange, Reynolds) but was always decent quality.
Any frame/fork with domed stay ends (as opposed to radiused stay ends) at the dropouts used the Japan-built subassemblies. All the Waterloo-built rear triangles and forks had radiused ends at the dropouts.
HTH…