Sorry, I should have said that the california press is a combination of the close grip and skullcrusher, whereas the JM press is actually more of a hybrid/modified skullcrusher.
The difference is that the JM press is supposed to be performed like a modified skullcrusher, whereas a california press is a hybrid, skullcrusher/closegrip press. I believe the JM is more effective to hitting all of the triceps heads, because it isolates and only works the triceps opposed to a...