My first "proper" bike was a Falcon Black Diamond bought new around 1976/77 I think. Definitely the same era transfers as yours.
They were a good, solid entry level bike, apart from the dodgy brake lever extensions so popular at the time.
Mine, I think, had a frame sticker saying it was made with Reynolds tubing so I always assummed 501. If it was 531 I believe they would have said so.
I covered a lot of miles on mine touring and commuting before it was stolen, never to be seen again.