Thought it was Pentax, but it's not. Not Contax/Yashica, either, at least no adj tab...
Actually, strongly resembles C/Y, except that large black piece at just before 6 o'clock in the photo.
It could be a Mamiya CS mount but I'm not sure. I have a couple of CS lenses coming later this week. I'll let you know. The CS mount is very close in looks and design to a k-mount.