Double Zero,
Not familiar with Herald's, Ambassadors etc., but I strongly suspect that they were. Similar aircraft from that era that I was associated with, F27,Viscount, Electra, early model DC9, B727 and B737 most certainly did have to comply.
Interesting that you note the Twin Otter (FHC6), as a light aircraft (<5700 Kg / 12500 Lb) the regulations did not require them to comply fully with FAR25 type requirements, but as we were operating them RPT within Australia, the regulatory authority (God bless 'em), required us (the airline) to prove that they did meet all of the FAR25 standards (except that they called it CAO 20.7.1B because they were Australian).
The one interesting concession granted to us by the same regulatory authority was that DC3 operations in New Guinea, due to very high airfield elevations and MUCH higher obstacles operated with "special" PK charts, which allowed for "all engines" operation only, treating the aircraft as one would a single engined aircraft. Jack Fry must have rolled over in his grave!
Best Regards,
Old Smokey