In my terminology which may not be strictly correct but serves its purpose, the entire joint of the forend iron's concave and action's convex surface is the 'knuckle'. Both parts form part of the knuckle so I would refer to them as the 'action knuckle' and the 'forend knuckle' meaning the metal in the immediate vicinity of that joint.
I am sure there is a more precise terminology but until someone publishes a definitive book on the subject....!
The London and Birmingham trade have different names for just about everything which is further complicated by the larger gunmaker's having their own peculiar terms.
All good clean fun!
