There was definitely a strike in Ferndale in 1921 - my late grandfather was in it. The following is taken from his obituary in the local paper:-
"Evan John came from Aberdare, and came to the Rhondda at a very early age and was employed for 46 years as a fireman at Nos 2 & 5 Pits. Well known as a singer, he was a member of the original "Eight Welsh Miners" during the 1921 strike."
I have not been able to find any information on the 8 miners!
Brian