I did actually see one web page that said that the Machen race has been going "since the 1970's", as well as one where 1979 was quoted. I have just looked in the Fellrunner magazines from 1977 and 1978, and there is no mention of Machen; however, I do remember seeing the name of Dave Francis (Westbury Harriers) at the top of results lists in the magazine around the time I started fell running, and a bit of googling reveals that he was the winner of the inaugural Snowdon Race in 1976.
The Autumn 1977 magazine actually has a good write-up of the Cilcain race, including the suggestion that it should only be Category C despite having 1000ft of climbing in 4 miles, due to too much road; which is interesting in the light of the Calendar data that you reproduced here.